Biosensor, biosensor chip and biosensor device

ABSTRACT

A biosensor includes a working electrode  101 , a counter electrode  102  opposing the working electrode  101 , a working electrode terminal  103  and a working electrode reference terminal  10  connected to the working electrode  101  by wires, and a counter electrode terminal  104  connected to the counter electrode  102  by a wire. By employing a structure with at least three electrodes, it is possible to assay a target substance without being influenced by the line resistance on the working electrode side.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/618,084, filed on Nov. 13, 2009, which is a continuation of U.S.application Ser. No. 12/360,639, filed on Jan. 27, 2009, now U.S. Pat.No. 8,388,820, which is a continuation of U.S. application Ser. No.10/488,325, filed on Mar. 2, 2004, now U.S. Pat. No. 7,540,947, which isa U.S. National Phase under 35 U.S.C. §371 of International ApplicationNo. PCT/JP2003/007593, filed on Jun. 16, 2003, claiming priority ofJapanese Patent Application Nos. JP 2002-193547, filed on Jul. 2, 2002,and JP 2002-304858, filed on Oct. 18, 2002, the entire contents of eachof which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a biosensor and a biosensor device forelectronically detecting the binding reaction of a biological substancesuch as an oligonucleotide, an antigen, an enzyme, a peptide, anantibody, a DNA fragment, an RNA fragment, glucose, lactic acid, orcholesterol.

BACKGROUND ART

Recently, the use of biosensing instruments using disposable samplepieces has been increasing each year, and it is expected to enablesimple and quick assay and analysis of a particular component in abiological body fluid such as blood, plasma, urine, or saliva, or thewhole set of proteins created in a cell at a certain point in time,i.e., a proteome. Moreover, individually-tailored medical treatments, inwhich individuals are treated and administered medicines according totheir SNP (acronym for Single Nucleotido Polymorphism) information, areexpected to be put into practice in the future by genetic diagnosisusing disposable DNA chips.

A conventional biosensor device for detecting the grape sugar level,i.e., the blood glucose level, of a blood sample described in JapanesePatent Application No. 11-509644 will now be described. Note that theterm “biosensor” as used herein refers to a disposable portion includinga detection section for detecting a biological substance, the term“biosensor chip” refers to a disposable portion including a biosensor, ameasurement circuit, etc., mounted on a substrate. Moreover, the term“biosensor device” refers to the entire device including a biosensor ora biosensor chip together with an analysis circuit and other parts.

FIG. 45 is a plan view illustrating the structure of a conventionalbiosensor. A biosensor 1122 illustrated in the figure includes a workingelectrode (anode) 1101, and a counter electrode (cathode) 1102 opposingthe working electrode 1101, and an assay reagent (not shown) made of anenzyme, a mediator, etc., corresponding to the assayed component isapplied on the working electrode 1101 and the counter electrode 1102.The working electrode 1101 is connected to a working electrode terminal1103 via a conductive line having a line resistance Rp1. Similarly, thecounter electrode 1102 is connected to a counter electrode terminal 1104via a conductive line having a line resistance Rm1.

FIG. 43 is a circuit diagram illustrating a portion of a conventionalbiosensor device. As illustrated in the figure, the conventionalbiosensor device has a structure in which the working electrode terminal1103 and the counter electrode terminal 1104 of the biosensor 1122illustrated in FIG. 45 are connected to a measurement circuit 1123. Forexample, the measurement circuit 1123 includes a base voltage source1117, a counter electrode voltage application section 1106, a workingelectrode voltage application section 1105 having an ammeter, and asignal processing circuit 1121. In the conventional biosensor device, aworking electrode base voltage Vpr1 generated from the base voltagesource 1117 is impedance-converted by the working electrode voltageapplication section 1105, and then a working electrode terminal voltageVp1 is supplied from the working electrode voltage application section1105 to the working electrode terminal 1103. At this time, the followingexpression holds.Vp1=Vpr1  (1)

Vp1 and Vpr1 in Expression (1) represent potential or voltage values.This also applies to Vm1 and Vmr below.

Moreover, a counter electrode base voltage Vmr1 generated from the basevoltage source 1117 is impedance-converted by the counter electrodevoltage application section 1106, and then a counter electrode terminalvoltage Vm1 is supplied from the counter electrode voltage applicationsection 1106 to the counter electrode terminal 1104. At this time, thefollowing expression holds.Vm1=Vmr1  (2)

The value of the current flowing out to the working electrode terminal1103 is measured by the working electrode voltage application section1105, and a working electrode current level signal s1120 indicating themeasurement result is supplied to the signal processing circuit 1121.The conventional biosensor device calculates the concentration of theassayed component based on the measured current level, and performs aresult displaying operation, or the like. Then, Expression (3) belowholds, where Vf1 is the electrode application voltage between theworking electrode terminal 1103 and the counter electrode terminal 1104.Vf1=Vpr1−Vmr1  (3)

Moreover, Vf is the sensor application voltage between the workingelectrode 1101 and the counter electrode 1102. Furthermore, when theblood sample is dripped onto the biosensor 1122, a charge according tothe grape sugar level thereof is generated at the working electrode 1101and the counter electrode 1102, whereby a current flows between theelectrodes. Then, the following expression holds, where If1 is thecurrent flowing on the working electrode 1101 side, and If2 is thecurrent flowing on the counter electrode 1102 side.If1=If2  (4)

The grape sugar level, i.e., the blood glucose level, is obtained bymeasuring the current If1 by the measurement circuit 1123.

FIG. 44 is a circuit diagram illustrating the conventional biosensordevice including specific circuit configuration examples of the workingelectrode voltage application section 1105 and the counter electrodevoltage application section 1106. As illustrated in the figure, theworking electrode voltage application section 1105 has a circuitconfiguration in which a feedback resistance Rf is negatively fed backto an operational amplifier, and the counter electrode voltageapplication section 1106 has an operational amplifier in anull-amplifier configuration, i.e., a buffer circuit configuration,thereby realizing the function described above.

FIG. 46 is a plan view illustrating the structure of a biosensor chip1124 in the conventional biosensor device illustrated in FIG. 44. Inthis example, only one pair of the biosensor 1122 and the measurementcircuit 1123 is formed on the same substrate.

Moreover, in the conventional biosensor device illustrated in FIG. 43,when the biosensor 1122 measures the blood glucose level, the followingexpression holds for the electrode application voltage Vf1 and thesensor application voltage Vf, which is the voltage difference between aworking electrode voltage Vp and a counter electrode voltage Vm, due tothe presence of the conductive line on the working electrode side havingthe line resistance Rp1 and the conductive line on the counter electrodehaving the line resistance Rm1.Vf=Vf1−(Rp1·If1+Rm1·If2)  (5)

Moreover, for the current If1 flowing on the working electrode 1101 sideand the current If2 flowing on the counter electrode 1102 side, thefollowing expression holds based on the Kirchhoff's law.If1=If2  (6)

Substituting Expression (3) and Expression (6) into Expression (5) andrearranging the expression yields the following expression.Vf=(Vpr1−Vmr1)−(Rp1+Rm1)·If1  (7)

Therefore, it can be seen that the electrode application voltage(Vpr1−Vmr1) supplied from the measurement circuit 1123 to the biosensor1122 drops by (Rp1+Rm1)·If1 to be equal to the sensor applicationvoltage Vf.

As described above, with the conventional biosensor device, it ispossible to easily assay the glucose level in blood.

PROBLEMS TO BE SOLVED BY THE INVENTION

The current If1 caused by the charge generated from the assay reagent isas shown in the following expression with respect to the grape sugarlevel Q and the sensor application voltage Vf.If1=f{Q,Vf}  (8)

Therefore, substituting Expression (4) into Expression (3) yields thefollowing expression.If1=f{Q,(Vpr1−Vmr1)−(Rp1+Rm1)·If1}  (9)

Thus, there was a problem in that the potential drop caused by the lineresistance Rp1 of the conductive line of the working electrode 1101 andthe line resistance Rm1 of the conductive line of the counter electrode1102 introduces an error in the current If1, thereby causing an error inthe final blood glucose level measured by the biosensor device.

In the prior art, a low-resistance noble metal material such as platinum(Pt), gold (Au), or silver (Ag), is used for the conductive line inorder to solve the problem. However, this causes another problem that itmakes the biosensor 1122 expensive. Since the biosensor portion isbasically disposable, it should desirably be as inexpensive as possible.Therefore, there is a strong demand for novel means for reducing theline resistance.

In addition, when the biosensor device is formed as the biosensor chip1124, a microfabrication technique is used for forming the conductivelines. Moreover, it is speculated that biosensor chips will be furtherminiaturized in the future. Then, the line resistance will be furtherincreased to cause substantial errors, significantly lowering the assayprecision of the biosensor device.

An object of the present invention is to solve the problems in the priorart as described above, and to provide a biosensor and a biosensordevice capable of performing an assay without being influenced by theline resistance of a conductive line.

DISCLOSURE OF THE INVENTION

A biosensor of the present invention includes: a working electrode to bein contact with an assayed fluid during an assay; a counter electrode tobe in contact with the assayed fluid during an assay, the counterelectrode opposing the working electrode with an interval therebetweenfor allowing a flow of the assayed fluid; a working electrode terminalconnected to the working electrode; a counter electrode terminalconnected to the counter electrode; and a reference terminal connectedto one or both of the working electrode and the counter electrode,through which substantially no current flows during an assay.

In this structure, the reference terminal is provided, whereby it ispossible to assay an assayed fluid without being influenced by theresistance between the working electrode and the working electrodeterminal or the resistance between the counter electrode and the counterelectrode terminal, thus realizing a biosensor capable of performing ahigh-precision assay.

A biological substance or a microorganism that changes a state of asubstance contained in the assayed fluid may be immobilized on at leastone of the working electrode and the counter electrode. Then, it ispossible to electrically detect a change in the assayed fluid through,for example, a catalytic reaction of an enzyme, an antigen-antibodyreaction, a binding reaction between genes, or the like. Thus, it ispossible to perform a more detailed assay than with an assay usingfluorescence.

The reference terminal may be connected to only one of the workingelectrode and the counter electrode. Then, it is possible to realize ahigh-precision assay with fewer components, as compared with a casewhere the reference terminal is provided both for the working electrodeand for the counter electrode. Therefore, the biosensor is particularlyeffective when a reduction in the manufacturing cost or a reduction inthe area is required.

The biosensor may further include: a first line connecting the workingelectrode to the working electrode terminal; a second line connectingthe working electrode or the counter electrode to the referenceterminal; and a third line connecting the counter electrode to thecounter electrode terminal. Then, it is possible to realize ahigh-precision assay by appropriately designing the pattern of theselines.

The reference terminal may include: a working electrode referenceterminal connected to the working electrode; and a counter electrodereference terminal connected to the counter electrode. Then, it ispossible to perform an assay with a higher precision than in a casewhere the reference terminal is provided only for one of the workingelectrode and the counter electrode.

The biosensor may further include: a fourth line connecting the workingelectrode to the working electrode terminal; a fifth line connecting theworking electrode to the working electrode reference terminal; a sixthline connecting the counter electrode to the counter electrode referenceterminal; and a seventh line connecting the counter electrode to thecounter electrode terminal, wherein at least two of the fourth line, thefifth line, the sixth line and the seventh line are provided indifferent wiring layers so as to at least partially overlap each otheras viewed from above. Then, it is possible to reduce the circuit area ascompared with a case where all the lines are provided in the same wiringlayer.

The first line and the second line may be provided in different wiringlayers. Then, it is possible to reduce the circuit area by, for example,arranging the lines so as to overlap each other.

Also when the second line and the third line are provided in differentwiring layers, it is possible to reduce the circuit area.

The working electrode, the counter electrode, the reference terminal,the working electrode terminal, the counter electrode terminal, thefirst line, the second line and the third line may be provided on asubstrate; and one of the working electrode terminal and the counterelectrode terminal may be provided on a reverse surface of thesubstrate. Then, it is possible to ensure an even larger wiring area,whereby it is possible to bring the resistance closer to the ideal valueof 0Ω.

Moreover, the working electrode terminal and the counter electrodeterminal may be provided in different wiring layers.

The third line may be provided so as to extend across a plurality ofwiring layers.

Moreover, in a case where the reference terminal is connected to onlyone of the working electrode and the counter electrode, the counterelectrode may have a generally-circular shape; and a portion of an innerperiphery of the working electrode may be circular with a substantiallyconstant distance from the counter electrode. Then, it is possible tomake the reaction of the assayed fluid uniform, while the electric fieldacting upon the first and counter electrodes is made uniform, therebyfurther improving the assay precision.

Alternatively, the working electrode may have a generally-circularshape; and a portion of an inner periphery of the counter electrode maybe circular with a substantially constant distance from the workingelectrode. Also in such a case, it is possible to make the reaction ofthe assayed fluid uniform, while the electric field acting upon thefirst and counter electrodes is made uniform, thereby further improvingthe assay precision.

A plurality of the working electrodes may be provided; and the counterelectrodes, each opposing one of the working electrodes, may beintegrated together. Then, it is possible to reduce the number ofelectrodes, thus reducing the manufacturing steps and the manufacturingcost. Moreover, since the cross-sectional area of the line connected tothe counter electrode terminal can be increased, whereby it is possibleto reduce the line resistance on the counter electrode terminal side.

A plurality of the counter electrodes may be provided; and the workingelectrodes, each opposing one of the working electrodes, may beintegrated together. Also in such a case, it is possible to reduce thenumber of electrodes, thus reducing the manufacturing cost.

A cross-sectional area of the third line may be greater than that of thefirst line. Then, the resistance of the third line can be brought closerto the ideal value of 0Ω.

A biosensor chip of the present invention includes: a biosensorincluding: a working electrode to be in contact with an assayed fluidduring an assay; a counter electrode to be in contact with the assayedfluid during an assay, the counter electrode opposing the workingelectrode with an interval therebetween for allowing a flow of theassayed fluid; a sensor section for holding the assayed fluid; a workingelectrode terminal connected to the working electrode; a counterelectrode terminal connected to the counter electrode; and a referenceterminal connected to one or both of the working electrode and thecounter electrode, through which substantially no current flows duringan assay, the biosensor being provided on a substrate; and a measurementcircuit connected to the biosensor and provided on a substrate.

In this structure, the reference terminal is connected to one or both ofthe working electrode and the counter electrode, whereby it is possibleto assay an assayed substance in the assayed fluid irrespective of theresistance value between the working electrode and the working electrodeterminal or the resistance value between the counter electrode and thecounter electrode terminal. Thus, it is possible to perform ahigh-precision assay.

A biological substance or a microorganism that changes a state of asubstance contained in the assayed fluid may be immobilized on at leastone of the working electrode and the counter electrode. Then, it ispossible to realize a quick and detailed assay.

The reference terminal may be connected to only one of the workingelectrode and the counter electrode. Then, it is possible to realize ahigh-precision assay with fewer components.

For example, the reference terminal may be connected to the workingelectrode; and the measurement circuit may include: a working electrodevoltage application section connected to the working electrode terminaland having an ammeter; a working electrode potential reference circuitconnected to the reference terminal; a counter electrode voltageapplication section connected to the counter electrode terminal; a basevoltage source for supplying a base voltage to each of the workingelectrode potential reference circuit and the counter electrode voltageapplication section; and a signal processing circuit for processing acurrent level signal output from the working electrode voltageapplication section according to a level of a current flowing throughthe working electrode terminal during an assay.

In such a case, it is preferred, for performing a high-precision assay,that the working electrode potential reference circuit generates asignal so that a voltage applied to the reference terminal issubstantially equal to the base voltage supplied to the workingelectrode potential reference circuit during an assay.

The reference terminal may be connected to the counter electrode; andthe measurement circuit may include: a working electrode voltageapplication section connected to the working electrode terminal; acounter electrode voltage application section connected to the counterelectrode terminal and having an ammeter; a counter potential referencecircuit connected to the reference terminal; a base voltage source forsupplying a base voltage to each of the counter electrode potentialreference circuit and the working electrode voltage application section;and a signal processing circuit for processing a current level signaloutput from the counter electrode voltage application section accordingto a level of a current flowing through the counter electrode terminalduring an assay.

In such a case, it is preferred that the counter electrode potentialreference circuit generates a signal so that a voltage applied to thereference terminal is substantially equal to the base voltage suppliedto the counter electrode potential reference circuit during an assay.

The reference terminal may be connected to the working electrode; andthe measurement circuit may include: a working electrode voltageapplication section connected to the working electrode terminal and thereference terminal and having an ammeter; a counter electrode voltageapplication section connected to the counter electrode terminal; a basevoltage source for supplying a base voltage to each of the workingelectrode voltage application section and the counter electrode voltageapplication section; and a signal processing circuit for processing acurrent level signal output from the working electrode voltageapplication section according to a level of a current flowing throughthe working electrode terminal during an assay. Then, it is possible toassay an assayed substance without providing the working electrodepotential reference circuit.

The reference terminal may be connected to the counter electrode; andthe measurement circuit may include: a working electrode voltageapplication section connected to the working electrode terminal; acounter electrode voltage application section connected to the counterelectrode terminal and the reference terminal and having an ammeter; abase voltage source for supplying a base voltage to each of the counterelectrode voltage application section and the working electrode voltageapplication section; and a signal processing circuit for processing acurrent level signal output from the counter electrode voltageapplication section according to a level of a current flowing throughthe counter electrode terminal during an assay. Then, it is possible toassay an assayed substance without providing the counter potentialreference circuit.

A working electrode reference terminal connected to the workingelectrode and a counter electrode reference terminal connected to thecounter electrode may be included. Then, it is possible to improve theassay precision as compared with a case where only the working electrodereference terminal or only the counter electrode reference terminal isprovided.

The measurement circuit may include: a working electrode voltageapplication section connected to the working electrode terminal and theworking electrode reference terminal; a counter electrode voltageapplication section connected to the counter electrode terminal and thecounter electrode reference terminal; a base voltage source forsupplying a base voltage to each of the counter electrode voltageapplication section and the working electrode voltage applicationsection; and a signal processing circuit for processing at least one ofa first current level signal output from the working electrode voltageapplication section according to a level of a current flowing throughthe working electrode terminal and a second current level signal outputfrom the counter electrode voltage application section according to alevel of a current flowing through the counter electrode terminal,during an assay.

In such a case, especially if the signal processing circuit processesboth the first current level signal and the second current level signal,it is possible to perform an assay by using two current level signals,thereby further improving the assay precision.

The substrate on which the biosensor is provided and the substrate onwhich the measurement circuit is provided may be the same substrate.Then, it is possible to simplify the manufacturing process.

The biosensor chip may further include a common substrate; and thesubstrate on which the biosensor is provided and the substrate on whichthe measurement circuit is provided may be mounted on the commonsubstrate. Then, it is possible to manufacture a biosensor chip even ina case where the substrate of the measurement circuit reacts with thebiological substance or the reagent immobilized on the first and counterelectrodes, or in a case where lines of the measurement circuit andlines of the biosensor cannot be integrated together into common lines,for example.

The substrate on which the biosensor is provided and the substrate onwhich the measurement circuit is provided may be stacked on each other.Then, it is possible to further reduce the area of the biosensor chipwhile reducing the manufacturing cost.

A plurality of the biosensors may be provided on the same substrate, andat least two of the biosensors may be connected to the same measurementcircuit; and a switch for turning ON/OFF a connection between each ofthe biosensors and the measurement circuit may be further providedbetween the working electrode terminal of the biosensor and themeasurement circuit, between the reference terminal of the biosensor andthe measurement circuit, and between the counter electrode terminal ofthe biosensor and the measurement circuit. Then, it is possible toreduce the number of measurement circuits required, whereby it ispossible to further reduce the chip area.

A plurality of the biosensors may be provided on the same substrate, andthe sensor sections of two of the biosensors may be provided adjacent toeach other. Then, it is possible to perform a plurality of assays at thesame time, while requiring a very small amount of sample.

A biosensor device of the present invention may include: a biosensorincluding: a working electrode to be in contact with an assayed fluidduring an assay; a counter electrode to be in contact with the assayedfluid during an assay, the counter electrode opposing the workingelectrode with an interval therebetween for allowing a flow of theassayed fluid; a sensor section for holding the assayed fluid; a workingelectrode terminal connected to the working electrode; a counterelectrode terminal connected to the counter electrode; and a referenceterminal connected to one or both of the working electrode and thecounter electrode, through which substantially no current flows duringan assay, the biosensor being provided on a substrate; and a measurementcircuit connected to the biosensor and provided on a substrate, whereinthe biosensor device has a function of assaying a concentration of anassayed substance contained in the assayed fluid from one or both of avalue of a current flowing through the working electrode terminal and avalue of a current flowing through the counter electrode terminal duringan assay. Then, it is possible to assay the target substance morequickly and with a higher precision over the prior art.

The reference terminal may be connected only to one of the workingelectrode and the counter electrode. Then, it is possible to realize anassay with a higher precision over the prior art, while reducing thenumber of components as compared with a case where the referenceterminal is provided both for the working electrode and for the counterelectrode.

The reference terminal may include: a working electrode referenceterminal connected to the working electrode; and a counter electrodereference terminal connected to the counter electrode; and themeasurement circuit may include: a working electrode voltage applicationsection connected to the working electrode terminal and the workingelectrode reference terminal; a counter electrode voltage applicationsection connected to the counter electrode terminal and the counterelectrode reference terminal; a base voltage source for supplying a basevoltage to each of the counter electrode voltage application section andthe working electrode voltage application section; and a signalprocessing circuit for processing at least one of a first current levelsignal output from the working electrode voltage application sectionaccording to a level of a current flowing through the working electrodeterminal and a second current level signal output from the counterelectrode voltage application section according to a level of a currentflowing through the counter electrode terminal, during an assay. Then,it is possible to perform an assay without being influenced by theresistance between the working electrode and the working electrodeterminal or the resistance between the counter electrode and the counterelectrode terminal, whereby it is possible to improve the assayprecision as compared with a case where the reference terminal isconnected to only one of the working electrode and the counterelectrode.

It is preferred, for an accurate assay, that a voltage applied to theworking electrode reference terminal is substantially equal to the basevoltage supplied to the working electrode voltage application sectionduring an assay; and a voltage applied to the counter electrodereference terminal is substantially equal to the base voltage suppliedto the counter electrode voltage application section during an assay.

The biosensor device may further include a circuit connected to themeasurement circuit for analyzing a signal output from the measurementcircuit. Then, it is possible to realize an accurate assay.

The biosensor and the measurement circuit may be provided on the samechip; and the chip can be replaced with another. Then, it is possible toprevent the contamination between samples, thereby simplifying the assayprocess.

The measurement circuit may further include a current level signalgeneration section for receiving the first current level signal and thesecond current level signal to output, to the signal processing circuit,a third current level signal representing a level of a current flowingbetween the working electrode and the counter electrode. Then, it ispossible to simplify the configuration of the signal processing circuitto be provided in a subsequent stage, thereby reducing the size of thedevice.

The reference terminal may be connected to the working electrode; andthe measurement circuit may include: a working electrode voltageapplication section connected to the working electrode terminal andhaving an ammeter; a working electrode potential reference circuitconnected to the reference terminal; a counter electrode voltageapplication section connected to the counter electrode terminal; a basevoltage source for supplying a base voltage to each of the workingelectrode potential reference circuit and the counter electrode voltageapplication section; and a signal processing circuit for processing acurrent level signal output from the working electrode voltageapplication section according to a level of a current flowing throughthe working electrode terminal during an assay.

It is preferred, for a high-precision assay, that the working electrodepotential reference circuit generates a signal so that a voltage appliedto the reference terminal is substantially equal to the base voltagesupplied to the working electrode potential reference circuit during anassay.

The reference terminal may be connected to the counter electrode; andthe measurement circuit may include: a working electrode voltageapplication section connected to the working electrode terminal; acounter electrode voltage application section connected to the counterelectrode terminal and having an ammeter; a counter potential referencecircuit connected to the reference terminal; a base voltage source forsupplying a base voltage to each of the counter electrode potentialreference circuit and the working electrode voltage application section;and a signal processing circuit for processing a current level signaloutput from the counter electrode voltage application section accordingto a level of a current flowing through the counter electrode terminalduring an assay.

In such a case, it is preferred, for a high-precision assay, that thecounter electrode potential reference circuit generates a signal so thata voltage applied to the reference terminal is substantially equal tothe base voltage supplied to the counter electrode potential referencecircuit during an assay.

The reference terminal may be connected to the working electrode; andthe measurement circuit may include: a working electrode voltageapplication section connected to the working electrode terminal and thereference terminal and having an ammeter; a counter electrode voltageapplication section connected to the counter electrode terminal; a basevoltage source for supplying a base voltage to each of the workingelectrode voltage application section and the counter electrode voltageapplication section; and a signal processing circuit for processing acurrent level signal output from the working electrode voltageapplication section according to a level of a current flowing throughthe working electrode terminal during an assay.

The reference terminal may be connected to the counter electrode; andthe measurement circuit may include: a working electrode voltageapplication section connected to the working electrode terminal; acounter electrode voltage application section connected to the counterelectrode terminal and the reference terminal and having an ammeter; abase voltage source for supplying a base voltage to each of the counterelectrode voltage application section and the working electrode voltageapplication section; a signal processing circuit for processing acurrent level signal output from the counter electrode voltageapplication section according to a level of a current flowing throughthe counter electrode terminal during an assay.

Moreover, the device as a whole may be disposable. Then, it is possibleto perform an assay more easily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating a portion of a biosensor deviceof the first embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a portion of the biosensordevice of the first embodiment including specific circuit configurationsof a working electrode voltage application section and a counterelectrode voltage application section.

FIG. 3 is a circuit diagram illustrating a portion of a biosensor deviceof the sixth embodiment of the present invention.

FIG. 4 is a circuit diagram illustrating a portion of the biosensordevice of the sixth embodiment including specific configurations of aworking electrode voltage application section and a counter electrodevoltage application section.

FIG. 5 is a circuit diagram illustrating a portion of a biosensor deviceof the seventh embodiment of the present invention.

FIG. 6 is a circuit diagram illustrating a portion of the biosensordevice of the seventh embodiment including specific configurations of aworking electrode voltage application section and a counter electrodevoltage application section.

FIG. 7 is a circuit diagram illustrating a portion of a biosensor deviceof the eighth embodiment of the present invention.

FIG. 8 is a circuit diagram illustrating a portion of the biosensordevice of the eighth embodiment including specific configurations of aworking electrode side potential reference voltage source and a counterelectrode side potential reference voltage source with an ammeter.

FIG. 9 is a plan view illustrating a biosensor of the first embodiment.

FIG. 10 is a diagram illustrating the biosensor of the first embodiment,where the conductive lines are multilayered.

FIG. 11 shows a plan view and a perspective view illustrating abiosensor of the second embodiment of the present invention.

FIG. 12 shows a plan view and a perspective view illustrating abiosensor of the third embodiment of the present invention.

FIG. 13 shows a plan view and a perspective view illustrating abiosensor of the fourth embodiment of the present invention.

FIG. 14 shows a plan view and a perspective view illustrating abiosensor of the fifth embodiment of the present invention.

FIG. 15 is a plan view illustrating a biosensor chip of the ninthembodiment of the present invention.

FIG. 16 is a plan view illustrating the first variation of the biosensorchip of the ninth embodiment.

FIG. 17 is a plan view illustrating the second variation of thebiosensor chip of the ninth embodiment.

FIG. 18 is a plan view illustrating the third variation of the biosensorchip of the ninth embodiment.

FIG. 19 is a plan view illustrating a biosensor chip of the tenthembodiment of the present invention.

FIG. 20 is a cross-sectional view illustrating the biosensor chip of thetenth embodiment.

FIG. 21 is a plan view illustrating a biosensor of the eleventhembodiment of the present invention.

FIG. 22 shows a plan view and a perspective view illustrating abiosensor of the twelfth embodiment of the present invention.

FIG. 23 is a plan view illustrating a biosensor chip of the thirteenthembodiment of the present invention.

FIG. 24 is a circuit diagram illustrating the configuration of abiosensor chip of the fourteenth embodiment of the present invention.

FIG. 25 is a plan view illustrating the biosensor chip of the fourteenthembodiment.

FIG. 26 is a plan view illustrating a biosensor chip of the fifteenthembodiment of the present invention.

FIG. 27 is a circuit configuration diagram illustrating a biosensordevice of the sixteenth embodiment of the present invention.

FIG. 28 is a circuit configuration diagram illustrating the biosensordevice of the sixteenth embodiment of the present invention.

FIGS. 29A-C show circuit diagrams each illustrating a working electrodevoltage application section and a counter electrode voltage applicationsection in the biosensor device of the sixteenth embodiment.

FIG. 30 is a circuit configuration diagram illustrating a biosensordevice of the seventeenth embodiment of the present invention.

FIG. 31 is a circuit configuration diagram illustrating a biosensordevice of the eighteenth embodiment of the present invention.

FIG. 32 is a plan view illustrating a biosensor of the nineteenthembodiment of the present invention.

FIG. 33 is a plan view illustrating a biosensor of the twentiethembodiment of the present invention.

FIG. 34 is a plan view illustrating a biosensor of the twenty-firstembodiment of the present invention.

FIG. 35 is a plan view illustrating a biosensor of the twenty-secondembodiment of the present invention.

FIG. 36 is a plan view illustrating a biosensor chip of the twenty-thirdembodiment of the present invention.

FIG. 37 is a plan view illustrating a biosensor chip of thetwenty-fourth embodiment of the present invention.

FIG. 38 is a plan view illustrating a biosensor chip of the twenty-fifthembodiment of the present invention.

FIG. 39 is a plan view illustrating a biosensor chip of the twenty-sixthembodiment of the present invention.

FIG. 40 is a circuit configuration diagram illustrating a measurementcircuit module of the twenty-sixth embodiment.

FIG. 41 is a plan view illustrating a biosensor chip of thetwenty-seventh embodiment of the present invention.

FIG. 42A is a structure diagram illustrating a biosensor chip of thetwenty-eighth embodiment of the present invention.

FIG. 42B is a cross-sectional view taken along line A-A in FIG. 42A.

FIG. 43 is a circuit diagram illustrating a portion of a conventionalbiosensor device.

FIG. 44 is a circuit diagram illustrating a portion of the conventionalbiosensor device including specific circuit configuration examples ofthe working electrode voltage application section and the counterelectrode voltage application section.

FIG. 45 is a plan view illustrating the structure of a conventionalbiosensor.

FIG. 46 is a plan view illustrating the structure of the biosensor chipin the conventional biosensor device illustrated in FIG. 44.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described withreference to the drawings. Note that like reference numerals denote likemembers throughout the various embodiments, and those members will notrepeatedly be described in detail.

First Embodiment

FIG. 1 is a circuit diagram illustrating a portion of a biosensor deviceof the first embodiment of the present invention, and FIG. 9 is a planview illustrating a biosensor of the first embodiment.

As illustrated in FIG. 9, a biosensor 15 of the present embodimentincludes a working electrode 101, a counter electrode 102 opposing theworking electrode 101, a working electrode terminal 103 and a workingelectrode reference terminal 10 both connected to the working electrode101, and a counter electrode terminal 104 connected to the counterelectrode 102. The connection of the working electrode 101 with theworking electrode terminal 103 and the working electrode referenceterminal 10, and the connection between the counter electrode 102 andthe counter electrode terminal 104 are made by conductive lines made ofa relatively inexpensive metal such as Al (aluminum) or Cu (copper).Moreover, the counter electrode 102 is connected to the counterelectrode terminal 104 via a conductive line having a sufficientcross-sectional area, whereby a line resistance Rm on the counterelectrode side can be regarded to be substantially 0Ω. Therefore, thecross-sectional area of the conductive line between the counterelectrode 102 and the counter electrode terminal 104 is greater thanthat of the conductive line between the working electrode 101 and theworking electrode terminal 103.

A sample containing an assayed substance such as glucose is introducedfrom outside into a reaction section including the working electrode 101and the counter electrode 102, and is assayed. Where glucose is assayed,for example, when a blood sample contacts glucose oxidase immobilized onthe working electrode 101 and the counter electrode 102, hydrogenperoxide is generated through a chemical reaction and electrons aregenerated. Then, a current flows between the electrodes, and the glucoselevel is assayed by measuring the current. Note that glucose oxidasedoes not need to be immobilized on both electrodes, but mayalternatively be immobilized on either the working electrode 101 or thecounter electrode 102.

Next, the biosensor device of the present embodiment illustrated in FIG.1 includes the biosensor 15 as described above and a measurement circuit16 connected to the working electrode reference terminal 10, the workingelectrode terminal 103 and the counter electrode terminal 104.

The measurement circuit 16 includes a working electrode potentialreference circuit 8 connected to the working electrode referenceterminal 10, a working electrode voltage application section 105connected to the working electrode terminal 103 and having an ammeter, acounter electrode voltage application section 106 connected to thecounter electrode terminal 104, a base voltage source 117 supplying theworking electrode base voltage Vpr1 and the counter electrode basevoltage Vmr1 to the working electrode potential reference circuit 8 andthe counter electrode voltage application section 106, respectively, anda signal processing circuit 121 connected to the working electrodevoltage application section 105.

FIG. 2 is a circuit diagram illustrating the biosensor device of thepresent embodiment including specific circuit configurations of theworking electrode voltage application section 105 and the counterelectrode voltage application section 106. As illustrated in the figure,the working electrode voltage application section 105 has a circuitconfiguration in which a feedback resistance Rf is negatively fed backto an operational amplifier, and the counter electrode voltageapplication section 106 has an operational amplifier in a null-amplifierconfiguration, i.e., a buffer circuit configuration, thereby realizingthe function described above.

A feature of the biosensor and the biosensor device of the presentembodiment is that the electrode connected to the working electrode 101is divided into two, i.e., the working electrode terminal 103 and theworking electrode reference terminal 10. The effect of this feature willbe described below.

First, in the biosensor device of the present embodiment, the counterelectrode base voltage Vmr1 generated from the base voltage source 117is impedance-converted by the counter electrode voltage applicationsection 106, and then the application voltage Vm1 is supplied from thecounter electrode voltage application section 106 to the counterelectrode terminal 104. At this time, the following expression holds.Vm1=Vmr1  (10)

Moreover, as the working electrode base voltage Vpr1 generated from thebase voltage source 117 and a working electrode reference terminalvoltage Vp2 from the working electrode reference terminal 10 are inputto the working electrode potential reference circuit 8, the workingelectrode potential reference circuit 8 generates a working electrodecontrol signal s13 so that the voltage difference therebetween is 0 V.The working electrode control signal voltage, which is the voltage ofthe working electrode control signal s13, is Vpr2. Then, therelationship of the following expression holds.Vp2=Vpr1  (11)Vp1=Vpr2  (12)

Moreover, the working electrode control signal voltage Vpr2 isimpedance-converted by the working electrode voltage application section105, and then the working electrode control signal voltage Vpr2 issupplied from the working electrode voltage application section 105 tothe working electrode terminal 103.

Next, in FIG. 1, the line resistance of the conductive line between theworking electrode 101 and the working electrode reference terminal 10 isRp2, and the working electrode reference terminal current flowingthrough the line is Ip2.

The input on the side of the working electrode potential referencecircuit 8 that is closer to the working electrode reference terminal 10is at a high input impedance, and the current flowing through theworking electrode reference terminal 10 is as shown in the followingexpression.Ip2=0  (13)

Therefore, the working electrode reference terminal voltage Vp2 and theworking electrode voltage Vp satisfy the following expression.Vp2=Vp  (14)

Therefore, from Expressions (10), (11), (13) and (14), the followingexpression holds for the sensor application voltage Vf.

$\begin{matrix}{\begin{matrix}{{Vf} = {{Vp} - {Vm}}} \\{= {{{Vp}\; 2} - \left( {{{Vm}\; 1} + {{If}\;{2 \cdot {Rm}}\; 1}} \right)}}\end{matrix}{{Now},{{{since}\mspace{14mu}{Rm}\; 1} = {0\Omega}},\begin{matrix}{{Vf} = {{{Vp}\; 2} - {{Vm}\; 1}}} \\{= {{{Vpr}\; 1} - {{Vmr}\; 1.}}}\end{matrix}}{{Therefore},{{Vf} = {{{Vpr}\; 1} - {{Vmr}\; 1.}}}}} & (15)\end{matrix}$

Thus, the voltage applied to the sensor application voltage Vf is alwaysconstant.

Therefore, in the biosensor device of the present embodiment,substituting Expression (15) into Expression (8) yields the followingexpression.If1=f{Q,(Vpr1−Vmr1)}Therefore, If1=f(Q).  (16)

Thus, there is no influence from the line resistance Rp1 of theconductive line of the working electrode 101, and no error occurs in thefinal blood glucose level measured by the biosensor device. The workingelectrode terminal voltage Vp1 is controlled by the working electrodepotential reference circuit 8 and the working electrode voltageapplication section 105 as shown in the following expression.Vp1=Vpr2Therefore, Vp1=Vpr1+Rp1·If1.  (17)

As described above, the biosensor device of the present embodimentincludes the biosensor having three electrodes, i.e., the workingelectrode terminal and the working electrode reference terminalbranching from the working electrode, and the counter electrode terminalconnected to the counter electrode, and the biosensor device of thepresent embodiment includes the working electrode potential referencecircuit 8 for generating the working electrode control signal s13 sothat the potential difference between a working electrode referencevoltage Vp2 and the working electrode base potential Vpr1 is 0, wherebyit can perform an assay without being influenced by the line resistance.Therefore, it is possible to perform an assay with a higher precisionthan the conventional biosensor device.

Moreover, since the assayed value is not influenced by the lineresistance, it is not necessary to use an expensive noble metal forlines as in the prior art, thus reducing the manufacturing cost.

Note that in the biosensor device of the present embodiment, the currentflowing through the working electrode voltage application section 105 isprocessed by the signal processing circuit 121 to calculate theconcentration of the assayed substance, and the calculated concentrationis displayed in a display section (not shown), or the like.

Moreover, in the biosensor device of the present embodiment, theconductive lines may be multilayered.

FIG. 10 is a diagram illustrating the biosensor of the presentembodiment, where the conductive lines are multilayered. In the exampleillustrated in the figure, the conductive line connecting the workingelectrode 101 to the working electrode reference terminal 10 is providedin a different layer than the conductive line connected to the workingelectrode terminal 103, i.e., the two conductive lines overlap eachother as viewed from above.

With such a structure, the area of the biosensor can be reduced fromthat of the biosensor illustrated in FIG. 9. Moreover, the reduction inthe area is advantageous in integrating biosensors for assayingdifferent substances together on a chip, and it may also lead to areduction in the manufacturing cost. For example, a biosensor formeasuring the grape sugar level and a biosensor for measuring the liverfunction indicators such as GOT and GTP may be multilayered together,whereby different assays can be done with a single blood sample, thusreducing the burden on the patient.

Moreover, multilayered lines may be used not only for the conductivelines for the working electrode but also for those for the counterelectrode. As biosensors are further miniaturized, the wiring area forthe counter electrode is reduced, thereby making it more difficult tobring the resistance close to 0. Therefore, by multilayering theconductive lines for the counter electrode by providing them in two ormore layers, the substantial wiring area can be increased, and theresistance value can be reduced.

Note that biosensor devices currently being sold widely are those forassaying glucose in which glucose oxidase, or the like, is immobilizedon the counter electrode and the working electrode. However, a differentsubstance may be immobilized on the electrodes in order to assay asubstance that binds to the immobilized substance, a substance thatreacts with the immobilized substance, or a substance that is decomposedor synthesized through a catalytic reaction with the immobilizedsubstance. For example, a single-stranded DNA may be immobilized on theelectrodes in order to detect a DNA or an RNA that pairs with theimmobilized DNA. As a DNA becomes double-stranded, the electricalconductivity thereof changes, whereby it can be detected electrically.This can be used in tests for diseases. For example, while a test forAIDS requires months before the antibody is generated, it is possible todetect an infection soon after the infection by performing an RNA assay.

Alternatively, a biological substance such as any of various enzymes maybe immobilized on the electrodes, or a microorganism may be immobilizedon the electrodes. For example, a microorganism assimilating carbondioxide may be immobilized in order to assay carbon dioxide in blood.Note that the term “biological substance” as used herein refers toproteins, amino acids, genes, and other organic matters in general,contained in the body of a living thing.

Moreover, it is possible to obtain a more detailed assayed value with anelectric assay than with a colorimetric assay using fluorescence.Therefore, the biosensor device of the present embodiment, capable ofperforming a precise assay, is useful in making a treatment plan.

Note that in the biosensor device of the present embodiment, only thebiosensor 15 or the biosensor with the measurement circuit 16 isdisposable. Alternatively, the device assembly including the displaysection and various other units may be disposable.

Note that while it is possible to employ a 4-terminal structure with acounter electrode reference terminal on the counter electrode side, thebiosensor of the present embodiment, as compared with one having a4-electrode structure, requires a smaller number of components, wherebyit is possible to reduce the cost and increase the wiring area. Incontrast, where a high precision is required, a 4-terminal biosensordevice is preferred. This will be described in detail in subsequentembodiments.

Note that in the biosensor of the present embodiment, the workingelectrode terminal and working electrode reference terminal arebranching from the working electrode. Alternatively, the conductive lineconnected to the working electrode terminal and the conductive lineconnected to the working electrode reference terminal may be a partiallyshared conductive line branching into two lines at a certain point.

Second Embodiment

FIG. 11 shows a plan view and a perspective view illustrating abiosensor 70 of the second embodiment of the present invention.

As illustrated in the figure, the biosensor of the present embodimentincludes the working electrode 101, the counter electrode 102 opposingthe working electrode 101, the working electrode reference terminal 10and the working electrode terminal 103 connected to the workingelectrode 101, and the counter electrode terminal 104 connected to thecounter electrode 102.

A feature of the biosensor of the present embodiment is that the counterelectrode terminal 104 connected to the counter electrode 102 extendsthrough the structure from the surface on which the working electrode101 is formed to the reverse surface, making the entire reverse surfacethe counter electrode terminal.

With such a structure, it is possible to further reduce the lineresistance value Rm1 on the counter electrode terminal side withoutchanging the size of the biosensor, thereby realizing a high-precisionbiosensor.

As described above, the biosensor of the present embodiment has a3-electrode structure with the working electrode, the working electrodereference terminal and the counter electrode, wherein the counterelectrode terminal extends through the structure from the surface onwhich the working electrode is formed to the reverse surface, making theentire reverse surface the counter electrode, whereby it is possible torealize a high-precision assay.

Third Embodiment

FIG. 12 shows a plan view and a perspective view illustrating abiosensor of the third embodiment of the present invention.

As illustrated in the figure, the biosensor of the present embodimentincludes the generally-circular counter electrode 102, the concentricring-shaped working electrode 101 surrounding the counter electrode 102with a constant interval therebetween, the working electrode referenceterminal 10 and the working electrode terminal 103 connected to theworking electrode 101, and the counter electrode terminal 104 connectedto the counter electrode 102. The counter electrode terminal 104 extendsthrough the structure from the surface on which the working electrode101 is formed to the reverse surface and extends across the entirereverse surface.

In the biosensor of the present embodiment, the working electrode 101 isformed in a concentric shape, whereby an enzyme and an assayed substancecan be reacted with each other in a uniform manner. Moreover, theelectric field acting upon the working electrode is made uniform,whereby it is possible to further improve the assay precision.

Moreover, the counter electrode terminal 104 is provided so as to extendacross the entire reverse surface, as in the second embodiment, therebyreducing the resistance on the counter electrode side and improving theassay precision.

Thus, with the biosensor of the present embodiment, it is possible toperform an assay with a significantly higher precision as compared withthe prior art.

Note that while the working electrode 101 is in a concentric shape inthe biosensor of the present embodiment, it may alternatively take apartial circular shape, e.g., by a semi-circular shape, for making theelectric field acting upon the working electrode uniform.

Fourth Embodiment

FIG. 13 shows a plan view and a perspective view illustrating abiosensor of the fourth embodiment of the present invention. Asillustrated in the figure, a biosensor 72 of the present embodimentincludes the working electrode 101, the counter electrode 102 providedso as to oppose the working electrode, the working electrode terminal103 connected to the working electrode 101 and provided so as to extendacross the entire reverse surface, and the counter electrode terminal104 and a counter electrode reference terminal 3 connected to thecounter electrode 102.

A 3-electrode structure may be obtained by providing a referenceelectrode on the counter electrode side, as in the present embodiment.Also in this case, it is possible to perform a high-precision assay asthe resistance of the conductive lines does not influence the assayedvalue, as described in the first embodiment. Thus, it is possible to usean inexpensive metal for the conductive lines, thereby reducing themanufacturing cost.

Note that in the example illustrated in FIG. 13, the working electrodeterminal 103 is formed across the entire reverse surface opposite to thesurface on which the working electrode 101 is formed, therebysuppressing the resistance value on the working electrode side to asignificantly small value. Note however that it is not necessary thatthe working electrode terminal 103 is provided on the reverse surface.

As described above, with the biosensor of the present embodiment, it ispossible to realize a high-precision assay. Moreover, since the problemof the line resistance due to miniaturization can be solved, the assayprecision does not decrease even when biosensors are furtherminiaturized.

Fifth Embodiment

FIG. 14 shows a plan view and a perspective view illustrating abiosensor of the fifth embodiment of the present invention. Asillustrated in the figure, a biosensor 73 of the present embodimentincludes the generally-circular working electrode 101, the counterelectrode 102 surrounding the working electrode 101 with a constantinterval therebetween, the working electrode reference terminal 10 andthe working electrode terminal 103 connected to the working electrode101 and provided on the reverse surface of the substrate, and thecounter electrode terminal 104 provided so as to extend across theentire upper surface of the substrate.

With the biosensor 73 of the present embodiment, the working electrode101 and the inner periphery of the counter electrode 102 surrounding theworking electrode 101 are concentric with each other, whereby an enzymeand an assayed substance can be reacted with each other in a uniformmanner. Moreover, the electric field acting upon the electrode is madeuniform, thereby further improving the assay precision.

In addition, since the counter electrode terminal 104 is provided so asto extend across the entire upper surface of the substrate, it ispossible to suppress the resistance Rm1 on the counter electrode side toa very small value. Therefore, the biosensor of the present embodimentprovides an improved assay precision.

Thus, it is possible to realize a biosensor capable of performing ahigh-precision assay by making the working electrode and the innerperiphery of the counter electrode concentric with each other and byproviding the counter electrode terminal 104 on the upper surface of thesubstrate.

Sixth Embodiment

FIG. 3 is a circuit diagram illustrating a portion of a biosensor deviceof the sixth embodiment of the present invention, and FIG. 4 is acircuit diagram illustrating a portion of the biosensor device of thepresent embodiment including specific configurations of a workingelectrode voltage application section 29 and a counter electrode voltageapplication section 28.

As illustrated in FIG. 3, the biosensor device of the present embodimentincludes the biosensor 15, and the measurement circuit 16 connected tothe biosensor 15.

The biosensor 15 includes the working electrode 101, the counterelectrode 102 opposing the working electrode 101, the working electrodereference terminal 10 and the working electrode terminal 103 connectedto the working electrode 101, and the counter electrode terminal 104connected to the counter electrode 102. The working electrode 101 isconnected to the working electrode reference terminal 10 and the workingelectrode terminal 103 by conductive lines each made of Cu, Al, or thelike.

Moreover, the measurement circuit 16 includes the working electrodevoltage application section 29 connected to the working electrodereference terminal 10 and the working electrode terminal 103 and havingan ammeter, the counter electrode voltage application section 28connected to the counter electrode terminal 104, the base voltage source117 supplying the working electrode base voltage Vpr1 to the workingelectrode voltage application section 29 and the counter electrode basevoltage Vmr1 to the counter electrode voltage application section 28,and the signal processing circuit 121 for processing the current inputto the working electrode voltage application section 29. The workingelectrode voltage application section 29 is a voltage-current conversioncircuit disclosed in Japanese Laid-Open Patent Publication No. 11-154833(U.S. Pat. No. 5,986,910).

In the biosensor device of the present embodiment, the counter electrodebase voltage Vmr1 generated from the base voltage source 117 isimpedance-converted by the counter electrode voltage application section28, and then the counter electrode terminal voltage Vm1 is applied fromthe counter electrode voltage application section 28. At this time, thefollowing expression holds.Vm1=Vmr1  (18)

Moreover, the working electrode base voltage Vpr1 and the workingelectrode reference terminal voltage Vp2 of the working electrodereference terminal 10 of the biosensor 15 are input to the workingelectrode voltage application section 29, and the working electrodecontrol signal voltage Vp1 is supplied to the working electrode terminal103 so that the voltage difference therebetween is substantially 0 V. Atthis time, the following expression holds.Vp2=Vpr1  (19)

The value of the current flowing out to the working electrode terminal103 is measured by the working electrode voltage application section 29,and a working electrode current level signal s120, which is the resultof the measurement, is supplied to the signal processing circuit 121.Based on the measured current level, the concentration of the assayedcomponent is calculated, and a result displaying operation, etc., isperformed.

Moreover, since the input of the working electrode reference terminal 10of the working electrode voltage application section 29 is at a highinput impedance, the current flowing through the reference electrode isas shown in the following expression.Ip2=0  (20)

Therefore, the working electrode reference terminal voltage Vp2 and theworking electrode voltage Vp satisfy the following expression.Vp2=Vp  (21)

Therefore, from Expressions (18), (19), (20) and (21), the followingexpression holds for the sensor application voltage Vf.

$\begin{matrix}{\begin{matrix}{{Vf} = {{Vp} - {Vm}}} \\{= {{{Vp}\; 2} - \left( {{{Vm}\; 1} + {{If}\;{2 \cdot {Rm}}\; 1}} \right)}}\end{matrix}{{Now},{{{since}\mspace{14mu}{Rm}\; 1} = {0\Omega}},\begin{matrix}{{Vf} = {{{Vp}\; 2} - {{Vm}\; 1}}} \\{= {{{Vpr}\; 1} - {{Vmr}\; 1.}}}\end{matrix}}{{Therefore},{{Vf} = {{{Vpr}\; 1} - {{Vmr}\; 1.}}}}} & (22)\end{matrix}$

Thus, the voltage applied to the sensor application voltage Vf is alwaysconstant. Therefore, in the present sixth embodiment, substitutingExpression (22) into Expression (8) yields the following expression.If1=f{Q,(Vpr1−Vmr1)}Therefore, If1=f(Q).  (23)

Therefore, there is no influence at all of the line resistance Rp1 ofthe conductive line connecting the working electrode 101 to the workingelectrode terminal 103, and no error is contained in the blood glucoselevel, for example, assayed by the biosensor device.

The working electrode terminal voltage Vp1 is controlled by the workingelectrode voltage application section 29 as shown in the followingexpression.Vp1=Vpr1+Rp1·If1  (24)

The biosensor device of the present embodiment is different from thebiosensor device of the first embodiment in that the biosensor device ofthe present embodiment includes the working electrode voltageapplication section 29 connected to both the working electrode referenceterminal 10 and the working electrode terminal 103. With this structure,it is possible to omit the capacitor for stabilizing the circuit,whereby it is possible to reduce the overall circuit area.

Thus, also in the structure where the working electrode voltageapplication section 29 functions also as the working electrode potentialreference circuit, it is possible to realize a high-precision biosensordevice that is not influenced by the line resistance on the workingelectrode side.

Note that an operational amplifier in which the negative input isconnected to the working electrode reference terminal 10, the positiveinput is connected to the base voltage source 117, and the output isconnected to the working electrode terminal 103, as illustrated in FIG.4, is shown as a specific example of the working electrode voltageapplication section 29. However, the present invention is not limited tothis configuration.

Seventh Embodiment

FIG. 5 is a circuit diagram illustrating a portion of a biosensor deviceof the seventh embodiment of the present invention, and FIG. 6 is acircuit diagram illustrating a portion of the biosensor device of thepresent embodiment including specific configurations of a workingelectrode voltage application section 19 and a counter electrode voltageapplication section 17.

As illustrated in FIG. 5, the biosensor device of the present embodimentincludes the biosensor 72, and the measurement circuit 16 connected tothe biosensor 72.

The biosensor 72 includes the working electrode 101, the counterelectrode 102 provided so as to oppose the working electrode 101, theworking electrode terminal 103 connected to the working electrode 101,and the counter electrode terminal 104 and the counter electrodereference terminal 3 connected to the counter electrode 102. Thecross-sectional area of the conductive line connecting the workingelectrode 101 to the working electrode terminal 103 is sufficientlylarge so that the line resistance can be made substantially 0Ω. Thebiosensor 72 includes the counter electrode reference terminal 3, asdoes the biosensor of the fourth embodiment.

Moreover, the measurement circuit 16 includes the working electrodevoltage application section 19 connected to the working electrodeterminal 103, the counter electrode voltage application section 17connected to the counter electrode terminal 104 and having an ammeter, acounter electrode potential reference circuit 1 connected to the counterelectrode reference electrode 3, the base voltage source 117 supplyingthe working electrode base voltage Vpr1 to the working electrode voltageapplication section 19 and the counter electrode base voltage Vmr1 tothe counter electrode potential reference circuit 1, and the signalprocessing circuit 121 for processing a counter electrode current levelsignal s18 output from the counter electrode voltage application section17 according to the input current.

In the biosensor device of the present embodiment, the working electrodebase voltage Vpr1 generated from the base voltage source 117 isimpedance-converted by the working electrode voltage application section19, and then the working electrode terminal voltage Vp1 is supplied fromthe working electrode voltage application section 19 to the workingelectrode terminal 103. At this time, the following expression holds.Vp1=Vpr1  (25)

Moreover, when the counter electrode base voltage Vmr1 generated fromthe base voltage source 117 and a working electrode reference terminalvoltage Vm2 are input to the counter electrode potential referencecircuit 1, the counter electrode potential reference circuit 1 generatesa counter electrode control signal s6 so that the voltage differencetherebetween is 0 V. The voltage of the counter electrode control signals6 (working electrode control signal voltage) is Vmr2. At this time, thefollowing expressions hold.Vm2=Vmr1  (26)Vm1=Vmr2  (27)

In FIG. 5, the current flowing out to the counter electrode terminal 104is measured by the counter electrode voltage application section 17, andthe result is supplied to the signal processing circuit 121 in the formof the counter electrode current level signal s18. Then, based on themeasured current level, the concentration of the assayed component iscalculated, and a result displaying operation, etc., is performed.

As in the first embodiment described above, the following expressionholds for the sensor application voltage Vf.

$\begin{matrix}{\begin{matrix}{{Vf} = {{Vp} - {Vm}}} \\{= {{{Vp}\; 1} - \left( {{{Vm}\; 2} + {{If}\;{1 \cdot {Rp}}\; 1}} \right)}}\end{matrix}{{Now},{{{since}\mspace{14mu}{Rp}\; 1} = {0\Omega}},\begin{matrix}{{Vf} = {{{Vp}\; 1} - {{Vm}\; 2}}} \\{= {{{Vpr}\; 1} - {{Vmr}\; 1.}}}\end{matrix}}{{Therefore},{{Vf} = {{{Vpr}\; 1} - {{Vmr}\; 1.}}}}} & (28)\end{matrix}$

Since Vpr1 and Vmr1 are constant, the sensor application voltage Vf isalways a constant value. Therefore, in the present third embodiment,substituting Expression (28) into Expression (8) yields the followingexpression.If2=f{Q,(Vpr1−Vmr1)}Therefore, If2=f(Q).  (29)

Therefore, the line resistance Rm1 of the conductive line on the counterelectrode 102 side does not influence If2 flowing through the counterelectrode terminal 104, whereby no error is contained in the final bloodglucose level measured by the biosensor device.

The counter electrode terminal voltage Vm1 is controlled by the counterelectrode potential reference circuit 1 and the counter electrodevoltage application section 17 as shown in the following expression.Vm1=Vmr2Therefore, Vm1=Vmr1−Rm1·If2.  (30)

Thus, it can be seen that with the seventh embodiment of the presentinvention, it is possible to perform a high-precision assay irrespectiveof the resistance of the conductive lines even with a 3-electrodestructure including the counter electrode terminal 104 and the counterelectrode reference electrode 3 on the counter electrode side. Inaddition, it requires a smaller number of components than in a casewhere four or more electrodes are provided, for example, whereby it ispossible to realize a low-cost, high-precision biosensor device.

Moreover, in the specific circuit example illustrated in FIG. 6, thecounter electrode voltage application section 17 has a circuitconfiguration in which a feedback resistance Rg20 is negatively fed backto an operational amplifier, and the working electrode voltageapplication section 19 has an operational amplifier in a null-amplifierconfiguration, i.e., a buffer circuit configuration. In this way, thecounter electrode voltage application section 17 and the workingelectrode voltage application section 19 provide the functions asdescribed above. Note that the counter electrode voltage applicationsection 17 and the working electrode voltage application section 19 mayalternatively have a different circuit configuration.

Eighth Embodiment

FIG. 7 is a circuit diagram illustrating a portion of a biosensor deviceof the eighth embodiment of the present invention, and FIG. 8 is acircuit diagram illustrating a portion of the biosensor device of thepresent embodiment including specific configurations of a workingelectrode voltage application section 31 and a counter electrode voltageapplication section 30.

As illustrated in FIG. 7, the biosensor device of the present embodimentincludes the biosensor 72, and the measurement circuit 16 connected tothe biosensor 72.

The configuration of the biosensor 72 is the same as that of the seventhembodiment.

The measurement circuit 16 includes the working electrode voltageapplication section 31, the counter electrode voltage applicationsection 30 connected to the counter electrode terminal 104 and thecounter electrode reference electrode 3 and having an ammeter, the basevoltage source 117 supplying the working electrode base voltage Vpr1 tothe working electrode voltage application section 31 and the counterelectrode base voltage Vmr1 to the counter electrode voltage applicationsection 30, and the signal processing circuit 121 for processing thecounter electrode current level signal s18 from the counter electrodevoltage application section 30.

The biosensor device of the present embodiment is different from theseventh embodiment in that the counter electrode potential referencecircuit 1 is absent, and the counter electrode voltage applicationsection 30 is connected to both the counter electrode terminal 104 andthe counter electrode reference electrode 3.

In the biosensor device of the present embodiment illustrated in FIG. 7,the counter electrode base voltage Vmr1 and the counter electrodereference electrode voltage Vm2 of the counter electrode referenceelectrode 3 are both input to the counter electrode voltage applicationsection 30, and the counter electrode control signal voltage Vmr2 issupplied to the counter electrode terminal 104 so that the voltagedifference therebetween is 0 V. At this time, the following expressionholds.Vm2=Vmr1  (31)

Moreover, the working electrode base voltage Vpr1 is impedance-convertedby the working electrode voltage application section 31, and then thevoltage Vp1 is supplied from the working electrode voltage applicationsection 31 to the working electrode terminal 103. At this time, thefollowing expression holds.Vp1=Vpr1  (32)

On the other hand, the current flowing out to the counter electrodeterminal 104 is measured by the counter electrode voltage applicationsection 30, and the counter electrode current level signal s18indicating the measurement result is supplied to the signal processingcircuit 121. Then, the device assembly calculates the concentration ofthe assayed component, and a result displaying operation, etc., isperformed.

As in the sixth embodiment described above, the following expressionholds for the sensor application voltage Vf.

$\begin{matrix}{\begin{matrix}{{Vf} = {{Vp} - {Vm}}} \\{= {{{Vp}\; 1} - \left( {{{Vm}\; 2} + {{If}\;{1 \cdot {Rp}}\; 1}} \right)}}\end{matrix}{{Now},{{{since}\mspace{14mu}{Rp}\; 1} = {0\Omega}},\begin{matrix}{{Vf} = {{{Vp}\; 1} - {{Vm}\; 2}}} \\{= {{{Vpr}\; 1} - {{Vmr}\; 1.}}}\end{matrix}}{{Therefore},{{Vf} = {{{Vpr}\; 1} - {{Vmr}\; 1.}}}}} & (33)\end{matrix}$

Thus, the sensor application voltage Vf is a constant voltage.

Therefore, substituting Expression (33) into Expression (8) yields thefollowing expression.If2=f{Q,(Vpr1−Vmr1)}Therefore, If2=f(Q).  (34)

Therefore, the blood glucose level measured by the biosensor device isnot influenced by the line resistance Rm1 of the conductive line on thecounter electrode 102 side, whereby no error occurs.

As described above, also when the counter electrode reference electrode3 and the counter electrode terminal 104 are both connected to thecounter electrode voltage application section 30, it is possible torealize a high-precision assay.

Moreover, in the specific circuit example illustrated in FIG. 8, thecounter electrode voltage application section 30 has an operationalamplifier in which the negative input is connected to the counterelectrode reference electrode 3, the positive input is connected to theworking electrode base voltage Vmr1, and the output is connected to theworking electrode terminal 103. This is a voltage-current conversioncircuit disclosed in Japanese Laid-Open Patent Publication No. 11-154833(U.S. Pat. No. 5,986,910). Note that the present invention is notlimited to this configuration.

Ninth Embodiment

A biosensor chip of the ninth embodiment of the present invention willnow be described.

FIG. 15 is a plan view illustrating the biosensor chip of the presentembodiment, FIG. 16 is a plan view illustrating the first variation ofthe biosensor chip of the present embodiment, FIG. 17 is a plan viewillustrating the second variation of the biosensor chip of the presentembodiment, and FIG. 18 is a plan view illustrating the third variationof the biosensor chip of the present embodiment.

As illustrated in FIG. 15, a biosensor chip 35 of the present embodimenthas a structure in which the biosensor of the first embodimentillustrated in FIG. 9 and the measurement circuit 16 are provided on thesame substrate. The biosensor and the measurement circuit 16 aremanufactured by using a microfabrication technique, and the conductiveline connecting the working electrode 101 to the working electrodeterminal 103 and the working electrode reference terminal 10 and theconductive line connecting the counter electrode 102 to the counterelectrode terminal 104 are formed as thin films. Moreover, theconductive line on the counter electrode side and that on the workingelectrode side are made of a relatively inexpensive metal such as Al orCu.

Moreover, the biosensor chip 35 of the present embodiment can bedetachable from the device assembly, and is disposable.

Thus, by integrating the biosensor and the measurement circuit 16together into a single chip, it is possible to reduce the size of theassay section, and it is possible to supply the biosensor chipinexpensively by using known mass-production techniques.

Note that when the conductive lines are formed by using amicrofabrication technique, the conductive lines are formed as thinfilms, thereby increasing the line resistances Rp1, Rm1 and Rp2.However, in the biosensor device of the present invention, ahigh-precision measurement is realized irrespective of the lineresistance, whereby it is possible to realize a biosensor chip that canbe used for a high-precision measurement and that is inexpensive.Moreover, since the size is small, the overall size of the biosensordevice can be reduced.

Note that not only the biosensor of the first embodiment, but also anyother biosensor described above, can be integrated together with themeasurement circuit into a chip.

Moreover, in the biosensor chip of the present embodiment, the commonsubstrate to be used may be any substrate, including a semiconductorsubstrate such as a silicon substrate, an SOI (Silicon on Insulator)substrate, an SOS (Silicon on Sapphire) substrate, an insulativesubstrate such as a glass substrate, etc. Note however that it isnecessary to choose a substrate that does not react with enzymes andreagents applied on the electrodes of the biosensor.

Moreover, a biosensor in which the conductive lines are multilayered asillustrated in FIG. 10 may also be provided on the common substrate withthe measurement circuit 16, as illustrated in FIG. 16. By multilayeringthe conductive lines, the area of the biosensor can be further reduced,whereby it is possible to manufacture an even smaller biosensor chip 37.

Alternatively, the biosensors illustrated in FIG. 11 and FIG. 12 may beprovided on the same substrate with the measurement circuit 16, asillustrated in FIG. 17. A biosensor chip 80 of this variation includes acommon substrate shared by the measurement circuit 16, and a substratewith a biosensor provided thereon and a substrate with the measurementcircuit 16 provided thereon are mounted on the common substrate. Acounter electrode terminal is provided so as to extend across the entirereverse surface of the substrate with the biosensor provided thereon.

Moreover, as illustrated in FIG. 18, even a biosensor in which twoelectrodes, i.e., the counter electrode reference electrode and thecounter electrode terminal illustrated in FIG. 13 and FIG. 14, areconnected to the counter electrode can be provided on the same commonsubstrate with the measurement circuit 16. Specifically, a substratewith the biosensor provided thereon and a substrate with the measurementcircuit 16 provided thereon are mounted on the common substrate.

Tenth Embodiment

FIG. 19 is a plan view illustrating a biosensor chip 40 of the tenthembodiment of the present invention, and FIG. 20 is a cross-sectionalview illustrating the biosensor chip 40 of the present embodiment.

As illustrated in FIG. 19 and FIG. 20, the biosensor chip 40 of thepresent embodiment includes a sensor chip 38 with a 3-electrodebiosensor provided thereon, a measurement circuit chip 43 with ameasurement circuit provided thereon, and a common substrate 60supporting the sensor chip 38 and the measurement circuit chip 43. Thecounter electrode terminal 104, the working electrode terminal 103 andthe working electrode reference terminal 10 of the biosensor areconnected to the measurement circuit chip 43 by wires 39.

In a case where the substrate with the measurement circuit providedthereon has a poor affinity to, or is reactive with, the assay reagent,etc., in the biosensor including an enzyme and a mediator, it isdifficult to provide such a substrate on the same common substrate withthe substrate with the measurement circuit 16 provided thereon, as inthe biosensor chip illustrated in FIG. 15. Therefore, a chip-on-chipstructure is employed, as in the present embodiment. In the biosensorchip 40 of the present embodiment, a substrate with a biosensor providedthereon and a substrate with a measurement circuit provided thereon canbe combined arbitrarily.

Moreover, there are cases where the same substance as that of the signalline of the measurement circuit 16 cannot be used for the conductivelines of the biosensor due to the type of the enzyme or mediatorcorresponding to the assayed component. Also in such a case, aconfiguration such as that of the present embodiment is useful, and itis possible with such a configuration to realize a sufficiently smallbiosensor chip.

With a chip-on-chip structure such as that of the present embodiment,any type of biosensor can be made into a small chip. Furthermore, sinceit does not involve any special processing step, it is possible torealize a low manufacturing cost.

Note that in the biosensor chip of the present embodiment, the sensorchip 38 and the measurement circuit chip 43 are arranged on the commonsubstrate 60. However, the measurement circuit chip 43 may be arrangeddirectly on the sensor chip 38 without using the common substrate 60.Alternatively, the biosensor chip may have a chip-on-chip structure inwhich the sensor chip 38 is arranged on the measurement circuit chip 43.

Moreover, while the sensor chip and the measurement circuit chip areconnected to each other by wires in the present embodiment, the uppersurface of the sensor chip and the upper surface of the measurementcircuit chip may alternatively be arranged so as to face each other andconnected to each other by solder bumps. Moreover, the chips mayalternatively be connected to each other by a ball grid array (acronymedto BGA). Alternatively, in a case where a pad or electrode passingthrough the substrate is provided, chips may be stacked on each otherand can still be connected to each other via the through electrode. Withthese methods, the signal transmission path is shortened, whereby theerror may be further reduced.

Eleventh Embodiment

FIG. 21 is a plan view illustrating a biosensor of the eleventhembodiment of the present embodiment.

As illustrated in FIG. 21, a biosensor 74 of the present embodimentincludes two 3-electrode biosensors formed on the same substrate, eachincluding the working electrode terminal 103, the working electrodereference terminal 10 and the counter electrode terminal 104, such asthat described in the first embodiment, for example, wherein the twocounter electrode terminals 104 are integrated together into a commonterminal. As the two counter electrode terminals 104 are integratedtogether into a common terminal, the number of electrodes is reduced,whereby it is possible to reduce the size, manufacturing cost, etc., ofthe biosensor.

Thus, by arranging two biosensors using different assay reagents made ofenzymes, mediators, etc., corresponding to different assayed components,it is possible to assay different factors at once, thus making itpossible to perform a plurality of tests at the same time and reducingthe burden on the patient. The number of types of biosensors to bemounted on a single biosensor device is not limited to any particularnumber as long as it is two or more. For practical purposes, it ispreferred, for example, to make it possible to perform, with a singlebiosensor chip, a plurality of tests that are necessary for diagnosing aparticular disease, or to make it possible to quickly perform a periodicmedical examination with a single biosensor chip. For this purpose,three or more biosensors may be formed on the same substrate, althoughFIG. 21 illustrates an example with only two biosensors formed on thesame substrate.

Moreover, the biosensor chip with biosensors mounted thereon isdetachable, whereby it is possible to selectively use differentbiosensor chips according to the purpose of the test while using thesame device assembly.

Note that while the counter electrode terminals are integrated togetheras a common terminal in the biosensor of the present embodiment, anyelectrodes that can be integrated together may be integrated togetherinto a common electrode. For example, by arranging two 3-electrodebiosensors in a symmetrical pattern, adjacent working electrodereference terminals 10 can be integrated together into a commonterminal.

Twelfth Embodiment

FIG. 22 shows a plan view and a perspective view illustrating abiosensor 75 of the twelfth embodiment of the present invention.

As illustrated in the figure, the biosensor 75 of the present embodimentincludes two biosensors of the second embodiment formed on the samesubstrate, wherein the two counter electrode terminals 104 areintegrated together into a common terminal. Thus, a common counterelectrode terminal 104 connected to two counter electrodes 102 isprovided so as to extend across the entire reverse surface of thebiosensor 75.

Also with a biosensor in which the counter electrode terminal isprovided so as to extend across the entire reverse surface, two or morebiosensors can be arranged together while integrating the counterelectrode terminals together into a common terminal, so that it ispossible to assay different assayed substances at the same time, whilereducing the number of electrodes and reducing the size. Moreover, asthe number of electrodes is reduced, the manufacturing process is alsosimplified. Moreover, by integrating the counter electrode terminals ofthe biosensors together into a common terminal, it is possible to ensurean even larger area and to bring the resistance value closer to theideal value of 0Ω.

Note that while two biosensors are arranged together in the presentembodiment, three or more biosensors may alternatively be arrangedtogether.

Moreover, also in a case where a plurality of biosensors are arrangedtogether, in each of which the counter electrode is provided so as toextend across the entire upper surface of the substrate as in the fifthembodiment, the counter electrode terminals can be integrated togetherinto a common terminal.

Moreover, also in a case where the conductive lines or electrodes on thecounter electrode side or the working electrode side are multilayered,two or more biosensors can be integrated into a single biosensor.

Thirteenth Embodiment

FIG. 23 is a plan view illustrating a biosensor chip 81 of thethirteenth embodiment of the present embodiment.

As illustrated in the figure, the biosensor chip 81 of the presentembodiment includes two biosensors and the measurement circuits 16connected to the respective biosensors, the biosensors each including asensor section 131 having three electrodes, i.e., the working electrodeterminal 103, the working electrode reference terminal 10 and thecounter electrode terminal 104. The biosensors and the measurementcircuits 16 are provided on the same substrate. Moreover, the counterelectrode terminals 104 of the adjacent biosensors are integratedtogether into a common terminal.

Each of the biosensors can assay a different substance, whereby it ispossible to perform a plurality of assays at the same time.

Note that while FIG. 23 illustrates an example where each biosensor andthe measurement circuit 16 are arranged next to each other, the presentinvention may employ an alternative structure, e.g., a structure where achip with a measurement circuit provided thereon is stacked on abiosensor. In such a case, the measurement circuit and the biosensor maybe connected to each other by a wire, a BGA or a through electrodepassing through the substrate.

Fourteenth Embodiment

FIG. 24 is a circuit diagram illustrating a biosensor chip 82 of thefourteenth embodiment of the present invention, and FIG. 25 is a planview illustrating the biosensor chip 82 of the present embodiment.

As illustrated in FIG. 24, the biosensor chip 82 of the presentembodiment includes a first biosensor 58, a second biosensor 59, and ameasurement circuit module 57 connected to the first biosensor 58 andthe second biosensor 59.

As illustrated in FIG. 25, the first biosensor 58 and the secondbiosensor 59 each include a working electrode terminal, a workingelectrode reference terminal and a counter electrode, and the counterelectrodes of the biosensors are connected to each other.

The measurement circuit module 57 includes the measurement circuit 16connected to the first biosensor 58 and the second biosensor 59, a firstgroup of switches 54 provided between the working electrode terminal andthe working electrode reference terminal of the first biosensor 58 andthe measurement circuit 16, a second group of switches 56 providedbetween the working electrode terminal and the working electrodereference terminal of the second biosensor 59 and the measurementcircuit 16, and a selection control circuit 52 for turning ON/OFF thefirst group of switches 54 and the second group of switches 56.

The selection control circuit 52 supplies a connection control signals53 to control the switching of the first group of switches 54 and aconnection control signal s55 to control the switching of the secondgroup of switches 56. Specifically, when an assay is performed by thefirst biosensor 58, the first group of switches 54 and the second groupof switches 56 are turned ON and OFF, respectively, whereas when anassay is performed by the second biosensor 59, the first group ofswitches 54 and the second group of switches 56 are turned OFF and ON,respectively.

With the biosensor chip 82 of the present embodiment, an assay can beperformed with only one measurement circuit for two biosensors, wherebyit is possible to assay a plurality of substances and to further reducethe chip area. Moreover, with this structure, it is possible to reducethe manufacturing cost.

In the biosensor chip of the present embodiment, the first group ofswitches 54 and the second group of switches 56 may have some on-stateresistance. However, since the on-state resistance is equivalentlyincluded in the line resistance of the conductive line of the biosensor,the assay precision is not lowered in the present circuit configuration.

Note that two biosensors are formed on the same substrate in thebiosensor chip of the present embodiment, three or more biosensors mayalternatively be formed on the same substrate. Moreover, since abiosensor to be measured can be selected by a switch, three or morebiosensors may be connected to one measurement circuit.

Moreover, while the first biosensor 58, the second biosensor 59 and themeasurement circuit module 57 are formed on the same substrate in thepresent embodiment, chips each having a biosensor or a measurementcircuit module thereon may alternatively be mounted on a commonsubstrate.

Alternatively, a plurality of chips may be stacked on one another andconnected together by a BGA, a through electrode or a wire.

Note that while the biosensor chip of the present embodiment includes a3-electrode biosensor having a working electrode terminal, a workingelectrode reference terminal and a counter electrode terminal, thebiosensor chip may alternatively include a 3-electrode biosensor havinga working electrode terminal, a counter electrode terminal and a counterelectrode reference electrode.

Fifteenth Embodiment

FIG. 26 is a plan view illustrating a biosensor chip 83 of the fifteenthembodiment of the present invention.

As illustrated in FIG. 26, the biosensor chip 83 of the presentembodiment includes two biosensors and a measurement circuit 50connected to the two biosensors on the same substrate, each biosensorincluding the working electrode terminal 103, the working electrodereference terminal 10, the counter electrode terminal 104, and thesensor section 131 for reacting an assayed fluid.

A feature of the biosensor chip 83 of the present embodiment is that thesensor sections 131 of the biosensors corresponding to different assayedcomponents are provided adjacent to each other. The reaction sectionincludes a counter electrode and a working electrode on which an assayreagent made of an enzyme, a mediator, etc., is applied.

In the biosensor chip of the present embodiment, the reaction sectionsof the two biosensors are adjacent to each other, whereby two differentassays can be performed only by dripping a single drop of blood sample.Thus, the structure of the dripping section of the biosensor issimplified. Moreover, it requires a very small amount of blood sample,thereby imposing a very little burden on the subject for bloodcollection.

Note that in the biosensor chip of the present embodiment, reactionsections of three or more different biosensors may alternatively beprovided adjacent to one another. Then, it is possible to perform threeor more different assays with a simple dripping section structure.Moreover, it is possible to reduce the amount of blood sample required.

Sixteenth Embodiment

While the embodiments described above are directed to a biosensorincluding three terminals, and a biosensor chip and a biosensor devicehaving the same, this and subsequent embodiments are directed toexamples where the biosensor includes four terminals.

FIG. 27 and FIG. 28 each show a circuit configuration of a biosensordevice of the sixteenth embodiment of the present invention. Thebiosensor device illustrated in these figures includes a biosensor 210of the present invention attached thereto, wherein a measurement circuit220 and the biosensor 210 are electrically connected to each other. Thestructure of the biosensor 210 will be described later. Note that inaddition to the biosensor 210 and the measurement circuit 220 asdescribed herein, the biosensor device includes a data analysis device,an assay result display section, etc., as necessary.

The measurement circuit 220 illustrated in FIG. 27 includes a workingelectrode voltage application section 221A for applying the voltage Vp1(corresponding to the “first working electrode voltage” of the presentinvention) to a working electrode terminal 213 a (corresponding to the“first working electrode terminal” of the present invention) of thebiosensor 210, a counter electrode voltage application section 222 forapplying the voltage Vm1 (corresponding to the “first counter electrodevoltage” of the present invention) to a counter electrode terminal 214 a(corresponding to the “first counter electrode terminal” of the presentinvention) of the biosensor 210, a base voltage source 223 for supplyinga voltage Vpr (corresponding to the “working electrode base voltage” ofthe present invention) and a voltage Vmr (corresponding to the “counterelectrode base voltage” of the present invention) to the workingelectrode voltage application section 221A and the counter electrodevoltage application section 222, respectively, and a signal processingcircuit 224 for processing a working electrode current level signal CV1output from the working electrode voltage application section 221A.

On the other hand, the measurement circuit 220 illustrated in FIG. 28includes a working electrode voltage application section 221 and acounter electrode voltage application section 222A instead of theworking electrode voltage application section 221A and the counterelectrode voltage application section 222, respectively, and the signalprocessing circuit 224 processes a counter electrode current levelsignal CV2 output from the counter electrode voltage application section222A.

The working electrode voltage application section 221 references thevoltage Vp2 of a working electrode reference terminal 213 b of thebiosensor 210. The working electrode voltage application section 221only references the voltage Vp2, and the input impedance is high,whereby the current Ip2 flowing through the working electrode referenceterminal 213 b is substantially zero. Therefore, there is no voltagedrop due to the resistance value Rp2 of the working electrode referenceterminal 213 b, and the voltage Vp2 and the voltage Vp (corresponding tothe “second working electrode voltage” of the present invention) can beconsidered to be equal to each other. Thus, essentially, the workingelectrode voltage application section 221 references the voltage Vp of aworking electrode 211 via the working electrode reference terminal 213b, and generates the voltage Vp1 so that the voltage Vp is matched withthe given voltage Vpr.

In addition to the function of the working electrode voltage applicationsection 221 described above, the working electrode voltage applicationsection 221A has a function of measuring the working electrode currentIf1 flowing through the working electrode terminal 213 a, and it outputsthe working electrode current level signal CV1 according to the measuredlevel of the working electrode current If1.

The counter electrode voltage application section 222 references thevoltage Vm2 of a counter electrode terminal 214 b (corresponding to the“second counter electrode terminal” of the present invention) of thebiosensor 210. The counter electrode voltage application section 222only references the voltage Vm2, and the input impedance is high,whereby a current Im2 flowing through the counter electrode terminal 214b is substantially zero. Therefore, there is no voltage drop due to aresistance value Rm2 of the counter electrode terminal 214 b, and thevoltage Vm2 and a voltage Vm (corresponding to the “second counterelectrode voltage” of the present invention) can be considered to beequal to each other. Thus, essentially, the counter electrode voltageapplication section 222 references the voltage Vm of a counter electrode212 via the counter electrode terminal 214 b, and generates the voltageVm1 so that the voltage Vm is matched with the given voltage Vmr.

In addition to the function described above, the counter electrodevoltage application section 222A has a function of measuring the counterelectrode current If2 flowing through the counter electrode terminal 214a, and it outputs the counter electrode current level signal CV2according to the measured level of the counter electrode current If2.

FIG. 29 shows some circuit examples of the working electrode voltageapplication sections 221 and 221A and the counter electrode voltageapplication sections 222 and 222A. The configurations of the circuitsillustrated in the figure will now be described successively.

FIG. 29( a) shows a circuit example of the working electrode voltageapplication section 221 or the counter electrode voltage applicationsection 222. The working electrode voltage application section 221 orthe counter electrode voltage application section 222 illustrated in thefigure has a configuration in which the output of a voltage referencecircuit 430, instead of the voltage Vpr or the voltage Vmr, is given toa counter electrode side voltage source 1106 of the conventionalmeasurement circuit 1123 illustrated in FIG. 44. The working electrodevoltage application section 221 will now be described as an example.

The voltage reference circuit 430 is an operational amplifier whoseinverting input terminal and non-inverting input terminal are given thevoltages Vp2 and Vpr, respectively. The voltage reference circuit 430outputs a voltage so that the voltage Vp2 and the voltage Vpr are equalto each other. An operational amplifier being a voltage source 420receives this voltage as its input, and outputs the voltage Vp1corresponding to the input voltage.

FIG. 29( b) shows a circuit example of the working electrode voltageapplication section 221A or the counter electrode voltage applicationsection 222A. The working electrode voltage application section 221A orthe counter electrode voltage application section 222A illustrated inthe figure has a configuration in which the output of the voltagereference circuit 430, instead of the voltage Vpr1 or the voltage Vmr1,is given to the voltage source 210 in the conventional biosensor deviceillustrated in FIG. 44. The working electrode voltage applicationsection 221A will now be described as an example.

An operational amplifier being the voltage reference circuit 430 outputsa voltage so that its inputs, i.e., the voltage Vp2 and the voltage Vpr,are equal to each other. The output voltage is given to thenon-inverting input terminal of the operational amplifier being thevoltage source 420. A resistive element is provided in the negativefeedback section of the operational amplifier, whereby the workingelectrode current level signal CV1 according to the level of the workingelectrode current If1 flowing through the resistive element is output.

FIG. 29( c) shows a circuit example of the working electrode voltageapplication section 221A or the counter electrode voltage applicationsection 222A. The working electrode voltage application section 221A orthe counter electrode voltage application section 222A illustrated inthe figure includes the voltage reference circuit 430 and avoltage-current conversion circuit 440. This circuit has a similarconfiguration to that of the voltage-current conversion circuitdisclosed in Japanese Laid-Open Patent Publication No. 11-154833 or U.S.Pat. No. 5,986,910, for example. The working electrode voltageapplication section 221A will now be described as an example.

The voltage reference circuit 430 outputs the voltage Vp1 so that itsinputs, i.e., the voltage Vp2 and the voltage Vpr, are equal to eachother. The voltage-current conversion circuit 440 receives as its inputa signal for controlling the output of the voltage reference circuit430, and outputs the working electrode current level signal CV1.

Next, the voltage applied to the biosensor 210 by the measurementcircuit 220 of the present embodiment, and the current measured by themeasurement circuit 220 will be described.

The voltage Vp1 is generated by the working electrode voltageapplication section 221 or 221A so that the voltage Vp and the voltageVpr are matched with each other, and the voltage Vp1 is applied to theworking electrode terminal 213 a. In this way, even if a voltage dropoccurs due to the resistance value Rp1 of the working electrode terminal213 a, the voltage Vp can be fixed to the voltage Vpr.

Similarly, the voltage Vm1 is generated by the counter electrode voltageapplication section 222 or 222A so that the voltage Vm and the voltageVmr are matched with each other, and the voltage Vm1 is applied to thecounter electrode terminal 214 a. In this way, even if a voltage dropoccurs due to the resistance value Rm1 of the counter electrode terminal214 a, the voltage Vm can be fixed to the voltage Vmr.

Therefore, the voltage Vf applied by the measurement circuit 220 betweenthe working electrode 211 and the counter electrode 212 of the biosensor210 is as shown in Expression (35) below.Vf=(Vpr−Vmr)  (35)

Then, from Expression (8) and Expression (35), a current If flowingthrough the biosensor 210 in response to the voltage application is asshown in Expression (36) below.If=f{Q,Vpr−Vmr}  (36)

Comparing Expression (35) and Expression (7) with each other shows thatthere is no voltage drop due to the line resistances Rp1 and Rm1 of theworking electrode terminal 213 a and the counter electrode terminal 214a in Expression (35). Thus, the voltage Vf applied between the workingelectrode 211 and the counter electrode 212 can be set to apredetermined value irrespective of the line resistances of the workingelectrode terminal 213 a and the counter electrode terminal 214 a of thebiosensor 210. Therefore, no error is contained in the current flowingthrough the biosensor 210. The current is measured as the workingelectrode current If1 or the counter electrode current If2 by theworking electrode voltage application section 221A or the counterelectrode voltage application section 222A, and converted to the workingelectrode current level signal CV1 or the counter electrode currentlevel signal CV2. The working electrode current level signal CV1 or thecounter electrode current level signal CV2 is processed by the signalprocessing circuit 224 to calculate the concentration of the assayedchemical substance.

As described above, according to the present embodiment, it is possibleto apply a predetermined voltage Vf between the working electrode 211and the counter electrode 212 irrespective of the line resistances ofthe working electrode terminal 213 a and the counter electrode terminal214 a of the biosensor 210. Thus, it is possible to measure an accuratecurrent level with no error, thereby improving the assay precision ofthe biosensor device. Particularly, in the biosensor device of thepresent embodiment, the working electrode reference terminal 213 b andthe counter electrode reference terminal 214 b are provided, whereby itis possible to further improve the assay precision as compared with acase where only one of the reference terminals is provided.

Note that in the working electrode voltage application section 221 orthe counter electrode voltage application section 222 illustrated inFIG. 29( a), the voltage source 420 may be omitted and the output of thevoltage reference circuit 430 may be used directly as the voltage Vp1 orthe voltage Vm1. Moreover, the voltage source 420 and the voltagereference circuit 430 may be implemented as an element other than anoperational amplifier. Such a change does not at all detract from theeffects of the present invention.

Moreover, where one of the working electrode 211 and the counterelectrode 212 is a first electrode and the other is a second electrode,the first voltage application section for applying the first voltage(e.g., the voltage Vp1) to the first terminal (e.g., the workingelectrode terminal 213 a) connected to the first electrode (e.g., theworking electrode 211) is a conventional voltage application section,while the second voltage application section for applying the secondvoltage (e.g., the voltage Vm1) to the second terminal (e.g., thecounter electrode terminal 214 a) connected to the second electrode(e.g., the counter electrode 212) is a voltage application section ofthe present embodiment (e.g., the counter electrode voltage applicationsection 222). The second voltage application section references thethird voltage (e.g., the voltage Vm) of the second electrode via thethird terminal (e.g., the counter electrode terminal 214 b) connected tothe second electrode, and generates the second voltage so that the thirdvoltage and a given base voltage (e.g., the voltage Vmr) are matchedwith each other. Thus, even when one of the working electrode voltageapplication section 221 or 221A and the counter electrode voltageapplication section 222 or 222A is omitted, it is possible to realize abiosensor device with an improved precision over the prior art.

Seventeenth Embodiment

FIG. 30 shows a circuit configuration of a biosensor device of theseventeenth embodiment of the present invention. A measurement circuit220A of the present embodiment includes the working electrode voltageapplication section 221A and the counter electrode voltage applicationsection 222A described in the sixteenth embodiment as means for applyinga voltage to the working electrode terminal 213 a and the counterelectrode terminal 214 a, respectively, of the biosensor 210, andprocesses the working electrode current level signal CV1 and the counterelectrode current level signal CV2 output from the voltage applicationsections to analyze the assayed chemical substance. The measurementcircuit 220A will now be described, where what has already beendescribed in the sixteenth embodiment will not be described again, andthe same reference numerals as those used in FIG. 27 and FIG. 28 will beused.

The working electrode voltage application section 221A measures thecurrent If1 flowing through the working electrode terminal 213 a as thecurrent flowing through the biosensor 210, and outputs the workingelectrode current level signal CV1. The working electrode voltageapplication section 221A may employ various configurations other thanthose of the circuits illustrated in FIG. 29( b) and FIG. 29( c).

The counter electrode voltage application section 222A measures thecurrent If2 flowing through the counter electrode terminal 214 a as thecurrent flowing through the biosensor 210, and outputs the counterelectrode current level signal CV2. The counter electrode voltageapplication section 222A may employ various configurations other thanthose of the circuits illustrated in FIG. 29( b) and FIG. 29( c).

A signal processing circuit 224A processes the working electrode currentlevel signal CV1 and the counter electrode current level signal CV2.While the signal to be processed is either the working electrode currentlevel signal CV1 or the counter electrode current level signal CV2 inthe sixteenth embodiment, these signals are both used in the presentembodiment, thereby doubling the amount of information on the currentflowing through the biosensor 210. Therefore, the S/N ratio can beimproved by about 6 db over the sixteenth embodiment.

As described above, according to the present embodiment, the assayprecision of the biosensor device can be further improved (by about 6 dbin terms of S/N ratio). Moreover, processing both the working electrodecurrent level signal CV1 and the counter electrode current level signalCV2 provides an effect of reducing the common-mode noise.

Eighteenth Embodiment

FIG. 31 shows a circuit configuration of a biosensor device of theeighteenth embodiment of the present invention. A measurement circuit220B of the present embodiment is similar to the measurement circuit220A of the seventeenth embodiment, but further includes a current levelsignal generation section 225. The measurement circuit 220B will now bedescribed, where what has already been described in the seventeenthembodiment will not be described again, and the same reference numeralsas those used in FIG. 30 will be used.

The current level signal generation section 225 receives, as its inputs,the working electrode current level signal CV1 and the counter electrodecurrent level signal CV2, and outputs a current level signal CVrepresenting the level of the current flowing through the biosensor 210.The current level signal generation section 225 can be implemented as adifferential signal converter, for example, as illustrated in FIG. 31.The differential signal converter adds together two input signals tooutput one signal. Thus, in the present embodiment, the current levelsignal CV is the result of adding together the working electrode currentlevel signal CV1 and the counter electrode current level signal CV2.

A signal processing circuit 224B is substantially the same in structureas the signal processing circuit 224 in the measurement circuit 220 ofthe sixteenth embodiment, and receives the current level signal CV asits input to calculate the concentration of the assayed chemicalsubstance.

As described above, according to the present embodiment, the workingelectrode current level signal CV1 and the counter electrode currentlevel signal CV2 are converted by the current level signal generationsection 225 into a single current level signal CV, whereby theconfiguration of the signal processing circuit 224B can be simplified ascompared with that of the seventeenth embodiment. Thus, it is possibleto reduce the size and the cost of the biosensor device. Note that thecurrent level signal generation section 225 can be implemented as anelement other than the differential signal converter illustrated in FIG.31.

Nineteenth Embodiment

FIG. 32 illustrates the structure of a biosensor of the nineteenthembodiment of the present invention. The biosensor 210 of the presentembodiment is used by the measurement circuits 220, 220A and 220B of thefirst to eighteenth embodiments described above, for example.

The biosensor 210 includes the working electrode terminal 213 a, 13 bextending from the working electrode 211, and the counter electrodeterminal 214 a and the counter electrode reference terminal 214 bextending from the counter electrode 212. Although not shown in thefigure, an assay reagent made of an enzyme, a mediator, etc., accordingto the assayed chemical substance is applied on the sensor sectionincluding the combination of the working electrode 211 and the counterelectrode 212. With the biosensor 210, it is possible to electronicallydetect the binding reaction between a pair of chemical substances, suchas an oligonucleotide, an antigen, an enzyme, a peptide, an antibody, aDNA fragment, an RNA fragment, glucose, lactic acid and cholesterol, orbetween molecular structures thereof.

The working electrode terminal 213 a is a terminal for voltageapplication from the measurement circuit (device assembly), and theworking electrode reference terminal 213 b is an electrode forreferencing the voltage. Note however that the position of the workingelectrode terminal 213 a and that of the working electrode referenceterminal 213 b may be switched around.

Similarly, the counter electrode terminal 214 a is a terminal forvoltage application from the measurement circuit, and the counterelectrode reference terminal 214 b is a terminal for referencing thevoltage. Again, the positions of the terminals may be switched around.

As a voltage is applied between the working electrode terminal and thecounter electrode terminal, the biosensor 210 outputs a currentaccording to the concentration of a particular chemical substancecontained in the body fluid such as blood placed on the sensor section.Then, the voltage at the working electrode 211 and the voltage at thecounter electrode 212 can be known by referencing the voltage at theworking electrode reference terminal and the voltage at the counterelectrode reference terminal, respectively.

As described above, according to the present embodiment, the workingelectrode terminal 213 a, the working electrode reference terminal 213b, the counter electrode terminal 214 a and the counter electrodereference terminal 214 b are provided in the biosensor 210, whereby itis possible to adjust the voltages applied to the working electrode 211and the counter electrode 212 while referencing the voltages at theworking electrode 211 and the counter electrode 212, and thus it ispossible to control the voltage applied between the working electrode211 and the counter electrode 212 to a predetermined value. Thus, it ispossible to eliminate the current error due to the line resistancewithout using a low-resistance noble metal for the lines connected tothe working electrode terminal 213 a, the working electrode referenceterminal 213 b, the counter electrode terminal 214 a and the counterelectrode reference terminal 214 b.

Note that while the biosensor of the present embodiment includes oneworking electrode terminal, one working electrode reference terminal,one counter electrode terminal and one counter electrode referenceterminal, the present invention is not limited to this, and more ofthese terminals may alternatively be provided. Specifically, two or moreof each of the working electrode terminal, the working electrodereference terminal, the counter electrode terminal and the counterelectrode reference terminal may alternatively be provided, and thenumber of working electrode terminals and the number of counterelectrode terminals may be different from each other. Moreover, thenumber of working electrode terminals and the number of workingelectrode reference terminals may be different from each other, and thenumber of counter electrode terminals and the number of counterelectrode reference terminals may be different from each other.

Moreover, where one of the working electrode 211 and the counterelectrode 212 is a first electrode and the other is a second electrode,the number of terminals for the first electrode (e.g., the workingelectrode 211) may be one, while the number of terminals for the secondelectrode (e.g., the counter electrode 212) is more than one. Also witha biosensor having such a structure, it is possible to reduce thecurrent error due to the line resistance over the prior art by using oneof the terminals connected to the second electrode for applying avoltage to the second electrode while using another one of the terminalsfor referencing the voltage at the second electrode.

Twentieth Embodiment

FIG. 33 illustrates the structure of a biosensor of the twentiethembodiment of the present invention. A biosensor 210A of the presentembodiment is similar to the biosensor 210 of the nineteenth embodiment,except that the electrodes are provided in a multilayered structure. Asillustrated in the figure, the working electrode terminal 213 a and theworking electrode reference terminal 213 b are layered over each other(so as to overlap each other as viewed from above), while the counterelectrode terminal 214 a and the counter electrode reference terminal214 b are layered over each other. In this way, it is possible to reducethe size of the biosensor, and further to reduce the cost thereof.

Note that while the working electrode terminal and the working electrodereference terminal are layered over each other, and the counterelectrode terminal and the counter electrode reference terminal arelayered over each other, in the present embodiment, the presentinvention is not limited to this. For example, it is possible to obtainan effect as described above by layering the working electrode terminaland the counter electrode terminal over each other, by layering theworking electrode terminal and the counter electrode reference terminalover each other, or by layering the working electrode reference terminaland the counter electrode terminal over each other.

Twenty-First Embodiment

FIG. 34 illustrates the structure of a biosensor of the twenty-firstembodiment of the present invention. In a biosensor 210B of the presentembodiment, two biosensors 210 of the nineteenth embodiment are formedon the same substrate. Although not shown in the figure, different assayreagents made of enzymes, mediators, etc., corresponding to differentassayed chemical substances are applied on the sensor section includingthe combination of a working electrode 211 a and a counter electrode 212a, and on the sensor section including the combination of a workingelectrode 211 b and a counter electrode 212 b. Thus, by providing aplurality of sensor sections on the same substrate, it is possible toassay a plurality of chemical substances at once, and it is possible torealize a biosensor of a higher performance and a lower price.

Note that while the biosensor 210B of the present embodiment includestwo sensor sections, it may alternatively include three or more sensorsections.

Twenty-Second Embodiment

FIG. 35 illustrates the structure of a biosensor of the twenty-secondembodiment of the present invention. A biosensor 210C of the presentembodiment is similar to the biosensor 210B of the twenty-firstembodiment, except that the counter electrodes 212 a and 212 b areintegrated into a single piece. The counter electrode 212 of thebiosensor 210C is both for the working electrode 211 a and for theworking electrode 211 b. In other words, the working electrodes 211 aand 211 b share a single counter electrode 212. Therefore, it ispossible to omit a counter electrode terminal 214 c and a counterelectrode reference terminal 214 d in the biosensor 210B, and it issufficient for the biosensor 210C to include one counter electrodeterminal 214 a and one counter electrode reference terminal 214 b. Thus,it is possible to further reduce the size of the biosensor.

Note that while two working electrode 211 a and 211 b share a singlecounter electrode 212 in the present embodiment, three or more workingelectrodes may be provided in the biosensor, the working electrodessharing a single counter electrode. Conversely, a plurality of counterelectrodes may be provided in the biosensor so as to share a singleworking electrode.

Twenty-Third Embodiment

FIG. 36 illustrates the structure of a biosensor chip of thetwenty-third embodiment of the present invention. A biosensor chip 230of the present embodiment includes a sensor section 231 and ameasurement circuit 232. An assay reagent made of an enzyme, a mediator,etc., according to the assayed chemical substance is applied on thesensor section 231, and as a voltage is applied thereto, the sensorsection 231 outputs a current according to the concentration of aparticular chemical substance contained in the body fluid such as bloodplaced thereon. The measurement circuit 232 applies a voltage to thesensor section 231, and measures the output current. Moreover, thesensor section 231 and the measurement circuit 232 are electricallyconnected to each other by working electrode lines 233 a and 233 b andthe counter electrode lines 234 a and 234 b.

The portion including the sensor section 231, the working electrodelines 233 a and 233 b and the counter electrode lines 234 a and 234 bhas a similar structure to that of the biosensor of the nineteenthembodiment. Specifically, the working electrode line 233 a and thecounter electrode line 234 a are used for applying voltages to theworking electrode 211 and the counter electrode 212, respectively,whereas the working electrode line 233 b and the counter electrode line234 b are used for referencing voltages at the working electrode 211 andthe counter electrode 212, respectively. Moreover, the measurementcircuit 232 has a similar circuit configuration to those of themeasurement circuits 220, 220A, 220B and 220C described in the first toeighteenth embodiments. Thus, the biosensor chip 230 includes abiosensor and a biosensor device of the present invention formed on asingle chip.

The working electrode lines 233 a and 233 b and the counter electrodelines 234 a and 234 b in the biosensor chip 230 are formed as thin filmsby a microfabrication process, whereby the resistance values thereof areincreased. However, according to the present embodiment, it is possibleto measure a current without being influenced by the resistance values,as described above. Therefore, it is possible to realize a biosensorchip that has a high precision and a very small size and is inexpensive.

Note that the substrate on which the biosensor chip 230 is formed may beof any material or structure as long as it is a substrate on which thesensor section 231 and the measurement circuit 232 can be formed, suchas a silicon substrate, a silicon-on-insulator substrate, asilicon-on-sapphire substrate, or a glass substrate.

Moreover, where one of the working electrode 211 and the counterelectrode 212 is a first electrode and the other is a second electrode,the first voltage application section for applying the first voltage(e.g., the voltage Vp1) to the first line (e.g., the working electrodeline 233 a) connecting the first electrode (e.g., the working electrode211) to the measurement circuit 232 is a conventional voltageapplication section, while the second voltage application section forapplying the second voltage (e.g., the voltage Vm1) to the second line(e.g., the counter electrode line 234 a) connecting the second electrode(e.g., the counter electrode 212) to the measurement circuit 232 is avoltage application section of the present embodiment (e.g., the counterelectrode voltage application section 222 or 222A). The second voltageapplication section references the third voltage (e.g., the voltage Vm)of the second electrode via the third line (e.g., the counter electrodeline 234 b) connecting the second electrode to the measurement circuit232, and generates the second voltage so that the third voltage and agiven base voltage (e.g., the voltage Vmr) are matched with each other.Thus, even if one of the working electrode voltage application section221 or 221A and the counter electrode voltage application section 222 or222A is omitted in the measurement circuit 232, it is possible torealize a biosensor chip that has a high precision and a very small sizeand is inexpensive.

Twenty-Fourth Embodiment

FIG. 37 illustrates the structure of a biosensor chip of thetwenty-fourth embodiment of the present invention. A biosensor chip 230Aof the present embodiment is similar to the biosensor chip 230 of thetwenty-third embodiment, except that the lines are provided in a layeredstructure. As illustrated in the figure, the working electrode lines 233a and 233 b are layered over each other, while the counter electrodelines 234 a and 234 b are layered over each other. Thus, it is possibleto reduce the size of the biosensor chip, and to reduce the pricethereof.

Note that while the working electrode lines are layered over each other,or the counter electrode lines are layered over each other, in thepresent embodiment, an effect similar to that described above can alsobe obtained by layering a working electrode line and a counter electrodeline over each other.

Twenty-Fifth Embodiment

FIG. 38 illustrates the structure of a biosensor chip of thetwenty-fifth embodiment of the present invention. A biosensor chip 230Bof the present embodiment is similar to the biosensor chip 230 of thetwenty-third embodiment, except that two sensor sections and twomeasurement circuits are formed on the same substrate. Although notshown in the figure, an assay reagent made of an enzyme, a mediator,etc., corresponding to the assayed chemical substance is applied on asensor section 231 a and a sensor section 231 b. Thus, by providing aplurality of sensor sections on the same substrate, it is possible tomeasure a plurality of chemical substances at once, whereby it ispossible to realize a biosensor chip of a higher performance and a lowerprice.

Note that while two sensor sections are provided in the biosensor chip230B in the present embodiment, three or more sensor sections mayalternatively be provided.

Twenty-Sixth Embodiment

FIG. 39 illustrates the structure of a biosensor chip of thetwenty-sixth embodiment of the present invention. A biosensor chip 230Cof the present embodiment is similar to the biosensor chip 230B of thetwenty-fifth embodiment, except that measurement circuits 232 a and 232b are integrated together into a single measurement circuit module 235.

FIG. 40 illustrates the circuit configuration of the measurement circuitmodule 235. The measurement circuit module 235 includes the measurementcircuit 232, switches 236 a, 236 b, 236 c and 236 d for turning ON/OFFthe connection between a first biosensor 431 a and the measurementcircuit 232, switches 236 e, 236 f, 236 g and 236 h for turning ON/OFFthe connection between a second biosensor 431 b and the measurementcircuit 232, and a selection control circuit 237 for controlling theoperation of the switches 236 a to 236 h. Note that the switches 236 ato 236 h and the selection control circuit 237 correspond to the“switching means” of the present invention.

The selection control circuit 237 closes/opens all of the switches 236 ato 236 d by using a control signal SEL1. Moreover, it closes/opens allof the switches 236 e to 236 h by using a control signal SEL2. Notehowever that the switches 236 a to 236 h will not be all closed at thesame time. Specifically, the selection control circuit 237 selects oneof the first biosensor 431 a and the second biosensor 431 b, andcontrols the switches 236 a to 236 h so that the selected biosensor andthe measurement circuit 232 are electrically connected to each other.

By providing the switches 236 a to 236 h between the biosensors 431 aand 431 b and the measurement circuit 232, the resistance valueincreases. However, according to the present invention, it is possibleto measure an accurate current level irrespective of the resistancevalue, as already described above.

As described above, according to the present embodiment, biosensors canbe switched to one another, whereby it is possible to reduce the numberof measurement circuits to be provided, as compared with the biosensorchip 230B of the twenty-fifth embodiment. Thus, it is possible tofurther reduce the size of the biosensor chip.

Note that while two control signals SEL1 and SEL2 are used to controlthe switches 236 a to 236 h in the present embodiment, the presentinvention is not limited to this. For example, the switches 236 a to 236h may be controlled by using only the control signal SEL1, or thebiosensors may be switched to one another by other methods.

Twenty-Seventh Embodiment

FIG. 41 illustrates the structure of a biosensor chip of thetwenty-seventh embodiment of the present invention. The circuitconfiguration of a biosensor chip 230D of the present embodiment issimilar to that of the biosensor chip 230C of the twenty-sixthembodiment. What is different from the biosensor chip 230C is that thesensor sections 231 a and 231 b are arranged adjacent to each other. Ifa plurality of sensor sections 231 a and 231 b are arranged adjacent toeach other, it is possible to analyze a plurality of chemical substancesby placing a body fluid sample such as blood only at a single pointrather than a plurality of points.

As described above, the present embodiment requires a very small amountof a body fluid sample such as blood, thereby reducing the burden on thepatient for blood collection, etc. Moreover, with the sensor sectionsbeing adjacent to each other, it is possible to simplify the structureof the section on which the sample is placed.

Note that by arranging the sensor sections adjacent to each other in abiosensor, it is possible to obtain an effect similar to that describedabove.

Twenty-Eighth Embodiment

FIG. 42 illustrates the structure of a biosensor chip of thetwenty-eighth embodiment of the present invention. A biosensor chip 240of the present embodiment is similar to the biosensor chip 230 of thetwenty-third embodiment, except that the biosensor chip 240 has achip-on-chip structure, where a sensor chip 241 and a measurementcircuit chip 242 are provided, instead of the sensor section 231 and themeasurement circuit 232, respectively, in different semiconductorintegrated circuits, with the chips being formed on the same substrate.The working electrode terminal 213 a, the working electrode referenceterminal 213 b, the counter electrode terminal 214 a and the counterelectrode reference terminal 214 b of the sensor chip 241 areelectrically connected to the measurement circuit chip 242 via wires 43.Note that FIG. 42( b) is a cross-sectional view taken along line A-A inFIG. 42( a).

In the twenty-third embodiment, if the assay reagent applied on thesensor section 231 in the biosensor chip 230 is not suited for thesubstrate material on which the measurement circuit chip 242 is formed,i.e., the substrate material of the biosensor chip 230, in terms of theaffinity or non-reactiveness, it will be very difficult to from thesensor section 231 and the measurement circuit chip 242 on the samesubstrate. Moreover, this is also true when the assay reagent is notsuited for the working electrode lines 233 a and 233 b and the counterelectrode lines 234 a and 234 b. However, with the biosensor chip 240 ofthe present embodiment, the sensor chip 241 and the measurement circuitchip 242 are formed in different semiconductor integrated circuits,whereby such a problem does not occur.

As described above, according to the present embodiment, a biosensorchip is formed in a chip-on-chip structure, whereby it is possible torealize biosensor chips with various assay reagents. This expands thevariety of objects that can be assayed by the biosensor chip.

Note that while the sensor chip 241 and the measurement circuit chip 242are arranged on a support substrate in the present embodiment, thepresent invention is not limited to this. Alternatively, the supportsubstrate may be omitted, in which case the measurement circuit chip 242may be arranged directly on the sensor chip 241, or the sensor chip 241may be arranged directly on the measurement circuit chip 242.

Moreover, while the sensor chip 241 and the measurement circuit chip 242are connected to each other by the wires 43, they may alternatively beconnected to each other by a ball grid array (BGA), or the like, insteadof the wires 43.

Moreover, the biosensor chip 240 of the present embodiment is obtainedby providing the biosensor chip 230 of the twenty-third embodiment in achip-on-chip structure, the present invention is not limited to this.For example, the biosensor chips 230A to 230D of the ninth totwenty-seventh embodiments, or biosensor chips of other structures, maybe provided in a chip-on-chip structure.

INDUSTRIAL APPLICABILITY

The biosensor device and the biosensor of the present invention cansuitably be used for assaying a biological substance, e.g., in a devicefor assaying the blood glucose level.

The invention claimed is:
 1. A biosensor device, comprising: a biosensor, comprising: a working electrode; a counter electrode; a working electrode terminal; a counter electrode terminal; a reference terminal; a first line connecting the working electrode to the working electrode terminal; a second line connecting the working electrode to the reference terminal; and a third line connecting the counter electrode to the counter electrode terminal; and a measurement circuit connected to the biosensor for performing an assay of a fluid to determine the concentration of a substance in the fluid, wherein at least one of the first line, the second line, and the third line is made of gold, and substantially no current flows through the second line during the assay.
 2. The biosensor device of claim 1, wherein a biological substance or a micro-organism that changes a state of a substance contained in a fluid to be assayed is placed on at least one of the working electrode and the counter electrode.
 3. The biosensor device of claim 1, wherein the biosensor is formed with at least two layers and the working electrode terminal and the counter electrode terminal are located on different layers.
 4. The biosensor device of claim 1, wherein the counter electrode is generally U shaped with a base and two fingers extending from the base, and the working electrode is located between the two fingers.
 5. The biosensor device of claim 1, wherein the working electrode has a generally-circular shape, and a portion of an inner periphery of the counter electrode is circular with a substantially constant distance from the working electrode.
 6. The biosensor device of claim 1, wherein the working electrode comprises a plurality of working electrodes.
 7. The biosensor device of claim 1, wherein the counter electrode comprises a plurality of counter electrodes.
 8. The biosensor device of claim 1, wherein the biosensor is disposable.
 9. The biosensor device of claim 1, wherein the working electrode, the counter electrode, the working electrode terminal, the counter electrode terminal, and the reference terminal are located on a substrate.
 10. The biosensor device of claim 1, wherein the working electrode, the counter electrode, the working electrode terminal, the counter electrode terminal, and the reference terminal are located on a substrate, the working electrode terminal is provided on one surface of the substrate and the counter electrode terminal is provided on a different surface of the substrate.
 11. The biosensor device of claim 1, wherein the counter electrode comprises a plurality of fingers.
 12. The biosensor device of claim 1, wherein the counter electrode comprises a plurality of fingers and the working electrode is disposed between the fingers of the counter electrode.
 13. The biosensor device of claim 1, wherein the working electrode and the counter electrode are disposed at one end of the biosensor and the working electrode terminal, the counter electrode terminal, and the reference terminal are disposed at an opposite end of the biosensor.
 14. The biosensor device of claim 1, wherein a cross-sectional area of the first line is greater than a cross-sectional area of the second line.
 15. The biosensor device of claim 1, wherein at least one of the lines is located in a layer of the biosensor that is different from another layer of the biosensor wherein at least one other of the lines is located.
 16. The biosensor device of claim 1, wherein at least one of the lines is located in a layer of the biosensor that is different from another layer of the biosensor wherein at least one other of the lines is located, and the lines at least partially overlap each other as viewed from above.
 17. The biosensor device of claim 1, wherein the first line is disposed between the second line and the third line on the biosensor.
 18. The biosensor device of claim 1, wherein at least one of the lines is formed by a microfabrication technique.
 19. The biosensor device of claim 1, wherein at least one of the lines comprises a partially shared line that branches into two lines.
 20. The biosensor device of claim 1, wherein at least one of the lines is formed as a thin film.
 21. The biosensor device of claim 1, wherein the working electrode, the counter electrode, the working electrode terminal, the counter electrode terminal, the reference terminal, the first line, the second line, and the third line are located on a substrate.
 22. The biosensor device of claim 1, wherein the width of the working electrode terminal is greater than that of the line connected to such terminal.
 23. The biosensor device of claim 1, wherein the width of the counter electrode terminal is greater than that of the line connected to such terminal.
 24. The biosensor device of claim 1, wherein the width of the reference terminal is greater than that of the line connected to such terminal.
 25. The biosensor device of claim 1, wherein the first line is connected to the working electrode at a first point and the second line is connected to the working electrode at a second point.
 26. The biosensor device of claim 1, wherein the first line is connected to the working electrode at a first point and the second line is connected to the working electrode at a second point, the first point located closer to the working electrode terminal than the second point.
 27. The biosensor device of claim 1, wherein at least one of the lines includes at least two line segments.
 28. The biosensor device of claim 1, wherein at least one of the lines includes at least two line segments that are positioned generally perpendicular to each other.
 29. The biosensor device of claim 1, wherein at least one of the lines includes at least two line segments that are positioned generally parallel to each other.
 30. The biosensor device of claim 1, wherein the second line is disposed outside the first line relative to the centerline that extends the length of the biosensor.
 31. The biosensor device of claim 1, wherein with respect to the relationship between the first line, the second line and the third line, the second line is adjacent the first line.
 32. The biosensor device of claim 1, wherein: the measurement circuit applies a voltage to the working electrode terminal during the assay; and a voltage at the working electrode is not equal to the voltage at the working electrode terminal.
 33. The biosensor device of claim 1, wherein: the measurement circuit applies a voltage to the counter electrode terminal during the assay; and a voltage at the counter electrode is not equal to the voltage at the counter electrode terminal.
 34. The biosensor device of claim 1, wherein: the measurement circuit applies a voltage to the working electrode terminal during the assay; and a voltage at the reference terminal is substantially equal to a voltage at the working electrode.
 35. The biosensor device of claim 1, wherein: the measurement circuit applies a voltage to the working electrode terminal and a voltage to the counter electrode terminal during the assay; and a current flowing through the first line is substantially equal to a current flowing through the third line.
 36. A biosensor device, comprising: a biosensor, comprising: a working electrode; a counter electrode; a working electrode terminal; a counter electrode terminal; a reference terminal; a first line connecting the working electrode to the working electrode terminal; a second line connecting the counter electrode to the reference terminal; and a third line connecting the counter electrode to the counter electrode terminal; and a measurement circuit connected to the biosensor for performing an assay of a fluid to determine the concentration of a substance in the fluid, wherein at least one of the first line, the second line, and the third line is made of gold, and substantially no current flows through the second line during the assay.
 37. The biosensor device of claim 36, wherein a biological substance or a micro-organism that changes a state of a substance contained in a fluid to be assayed is placed on at least one of the working electrode and the counter electrode.
 38. The biosensor device of claim 36, wherein the biosensor is formed with at least two layers and the working electrode terminal and the counter electrode terminal are located on different layers.
 39. The biosensor device of claim 36, wherein the counter electrode is generally U shaped with a base and two fingers extending from the base, and the working electrode is located between the two fingers.
 40. The biosensor device of claim 36, wherein the working electrode has a generally-circular shape, and a portion of an inner periphery of the counter electrode is circular with a substantially constant distance from the working electrode.
 41. The biosensor device of claim 36, wherein the working electrode comprises a plurality of working electrodes.
 42. The biosensor device of claim 36, wherein the counter electrode comprises a plurality of counter electrodes.
 43. The biosensor device of claim 36, wherein the biosensor is disposable.
 44. The biosensor device of claim 36, wherein the working electrode, the counter electrode, the working electrode terminal, the counter electrode terminal, and the reference terminal are located on a substrate.
 45. The biosensor device of claim 36, wherein the working electrode, the counter electrode, the working electrode terminal, the counter electrode terminal, and the reference terminal are located on a substrate, the working electrode terminal is provided on one surface of the substrate and the counter electrode terminal is provided on a different surface of the substrate.
 46. The biosensor device of claim 36, wherein the counter electrode comprises a plurality of fingers.
 47. The biosensor device of claim 36, wherein the counter electrode comprises a plurality of fingers and the working electrode is disposed between the fingers of the counter electrode.
 48. The biosensor device of claim 36, wherein the working electrode and the counter electrode are disposed at one end of the biosensor and the working electrode terminal, the counter electrode terminal, and the reference terminal are disposed at an opposite end of the biosensor.
 49. The biosensor device of claim 36, wherein a cross-sectional area of the third line is greater than a cross-sectional area of the second line.
 50. The biosensor device of claim 36, wherein at least one of the lines is located in a layer of the biosensor that is different from another layer of the biosensor wherein at least one other of the lines is located.
 51. The biosensor device of claim 36, wherein at least one of the lines is located in a layer of the biosensor that is different from another layer of the biosensor wherein at least one other of the lines is located, and the lines at least partially overlap each other as viewed from above.
 52. The biosensor device of claim 36, wherein the third line is disposed between the second line and the first line on the biosensor.
 53. The biosensor device of claim 36, wherein at least one of the lines is formed by a microfabrication technique.
 54. The biosensor device of claim 36, wherein at least one of the lines comprises a partially shared line that branches into two lines.
 55. The biosensor device of claim 36, wherein at least one of the lines is formed as a thin film.
 56. The biosensor device of claim 36, wherein the working electrode, the counter electrode, the working electrode terminal, the counter electrode terminal, the reference terminal, the first line, the second line, and the third line are located on a substrate.
 57. The biosensor device of claim 36, wherein the width of the working electrode terminal is greater than that of the line connected to such terminal.
 58. The biosensor device of claim 36, wherein the width of the counter electrode terminal is greater than that of the line connected to such terminal.
 59. The biosensor device of claim 36, wherein the width of the reference terminal is greater than that of the line connected to such terminal.
 60. The biosensor device of claim 36, wherein the second line is connected to the counter electrode at a first point and the third line is connected to the counter electrode at a second point.
 61. The biosensor device of claim 36, wherein the second line is connected to the counter electrode at a first point and the third line is connected to the counter electrode at a second point, the second point located closer to the counter electrode terminal than the first point.
 62. The biosensor device of claim 36, wherein at least one of the lines includes at least two line segments.
 63. The biosensor device of claim 36, wherein at least one of the lines includes at least two line segments that are positioned generally perpendicular to each other.
 64. The biosensor device of claim 36, wherein at least one of the lines includes at least two line segments that are positioned generally parallel to each other.
 65. The biosensor device of claim 36, wherein the second line is disposed outside the third line relative to the centerline that extends the length of the biosensor.
 66. The biosensor device of claim 36, wherein with respect to the relationship between the first line, the second line and the third line, the second line is adjacent the third line.
 67. The biosensor device of claim 36, wherein: the measurement circuit applies a voltage to the working electrode terminal during the assay; and a voltage at the working electrode is not equal to the voltage at the working electrode terminal.
 68. The biosensor device of claim 36, wherein: the measurement circuit applies a voltage to the counter electrode terminal during the assay; and a voltage at the counter electrode is not equal to the voltage at the counter electrode terminal.
 69. The biosensor device of claim 36, wherein: the measurement circuit applies a voltage to the counter electrode terminal during the assay; and a voltage at the reference terminal is substantially equal to a voltage at the counter electrode.
 70. The biosensor device of claim 36, wherein: the measurement circuit applies a voltage to the working electrode terminal and a voltage to the counter electrode terminal during the assay; and a current flowing through the first line is substantially equal to a current flowing through the third line.
 71. A biosensor device, comprising: a biosensor, comprising: a working electrode; a counter electrode; a working electrode terminal; a counter electrode terminal; a first reference terminal; a second reference terminal; a first line connecting the working electrode to the working electrode terminal; a second line connecting the working electrode to the first reference terminal; a third line connecting the counter electrode to the counter electrode terminal; and a fourth line connecting the counter electrode to the second reference terminal; and a measurement circuit connected to the biosensor for performing an assay of a fluid to determine the concentration of a substance in the fluid, wherein at least one of the first line, the second line, the third line, and the fourth line is made of gold; substantially no current flows through the second line during the assay; and substantially no current flows through the fourth line during the assay.
 72. The biosensor device of claim 71, wherein a biological substance or a micro-organism that changes a state of a substance contained in a fluid to be assayed is placed on at least one of the working electrode and the counter electrode.
 73. The biosensor device of claim 71, wherein the biosensor is formed with at least two layers and the working electrode terminal and the counter electrode terminal are located on different layers.
 74. The biosensor device of claim 71, wherein the counter electrode is generally U shaped with a base and two fingers extending from the base, and the working electrode is located between the two fingers.
 75. The biosensor device of claim 71, wherein the working electrode has a generally-circular shape, and a portion of an inner periphery of the counter electrode is circular with a substantially constant distance from the working electrode.
 76. The biosensor device of claim 71, wherein the working electrode comprises a plurality of working electrodes.
 77. The biosensor device of claim 71, wherein the counter electrode comprises a plurality of counter electrodes.
 78. The biosensor device of claim 71, wherein the biosensor is disposable.
 79. The biosensor device of claim 71, wherein the counter electrode comprises a plurality of fingers.
 80. The biosensor device of claim 71, wherein the counter electrode comprises a plurality of fingers and the working electrode is disposed between the fingers of the counter electrode.
 81. The biosensor device of claim 71, wherein the working electrode and the counter electrode are disposed at one end of the biosensor and the working electrode terminal, the counter electrode terminal, the first reference terminal, and the second reference terminal are disposed at an opposite end of the biosensor.
 82. The biosensor device of claim 71, wherein a cross-sectional area of at least one of the first and third lines is greater than a cross-sectional area of at least one of the second and fourth lines.
 83. The biosensor device of claim 71, wherein at least one of the lines is located in a layer of the biosensor that is different from another layer of the biosensor wherein at least one other of the lines is located.
 84. The biosensor device of claim 71, wherein at least one of the lines is located in a layer of the biosensor that is different from another layer of the biosensor wherein at least one other of the lines is located, and the lines at least partially overlap each other as viewed from above.
 85. The biosensor device of claim 71, wherein the first line is disposed between the second line and the third line on the biosensor.
 86. The biosensor device of claim 71, wherein the third line is disposed between the first line and the fourth line.
 87. The biosensor device of claim 71, wherein at least one of the lines is formed by a microfabrication technique.
 88. The biosensor device of claim 71, wherein at least one of the lines comprises a partially shared line that branches into two lines.
 89. The biosensor device of claim 71, wherein at least one of the lines is formed as a thin film.
 90. The biosensor device of claim 71, wherein the working electrode, the counter electrode, the working electrode terminal, the counter electrode terminal, the first reference terminal, the second reference terminal, the first line, the second line, the third line, and the fourth line are located on a substrate.
 91. The biosensor device of claim 71, wherein the width of the working electrode terminal is greater than that of the line connected to such terminal.
 92. The biosensor device of claim 71, wherein the width of the counter electrode terminal is greater than that of the line connected to such terminal.
 93. The biosensor device of claim 71, wherein the width of the first reference terminal is greater than that of the line connected to such terminal.
 94. The biosensor device of claim 71, wherein the width of the second reference terminal is greater than that of the line connected to such terminal.
 95. The biosensor device of claim 71, wherein the first line is connected to the working electrode at a first point and the second line is connected to the working electrode at a second point.
 96. The biosensor device of claim 71, wherein the third line is connected to the counter electrode at a first point and the fourth line is connected to the counter electrode at a second point.
 97. The biosensor device of claim 71, wherein the first line is connected to the working electrode at a first point and the second line is connected to the working electrode at a second point, the first point located closer to the working electrode terminal than the second point.
 98. The biosensor device of claim 71, wherein the third line is connected to the counter electrode at a first point and the fourth line is connected to the counter electrode at a second point, the first point located closer to the counter electrode terminal than the second point.
 99. The biosensor device of claim 71, wherein at least one of the lines includes at least two line segments.
 100. The biosensor device of claim 71, wherein at least one of the lines includes at least two line segments that are positioned generally perpendicular to each other.
 101. The biosensor device of claim 71, wherein at least one of the lines includes at least two line segments that are positioned generally parallel to each other.
 102. The biosensor device of claim 71, wherein the second line is disposed outside the first line relative to the centerline that extends the length of the biosensor.
 103. The biosensor device of claim 71, wherein the fourth line is disposed outside the third line relative to the centerline that extends the length of the biosensor.
 104. The biosensor device of claim 71, wherein: the measurement circuit applies a voltage to the working electrode terminal during the assay; and a voltage at the working electrode is not equal to the voltage at the working electrode terminal.
 105. The biosensor device of claim 71, wherein: the measurement circuit applies a voltage to the counter electrode terminal during the assay; and a voltage at the counter electrode is not equal to the voltage at the counter electrode terminal.
 106. The biosensor device of claim 71, wherein: the measurement circuit applies a voltage to the working electrode terminal and a voltage to the counter electrode terminal during the assay; a voltage at the first reference terminal is substantially equal to a voltage at the working electrode during the assay; and a voltage at the second reference terminal is substantially equal to a voltage at the counter electrode during the assay.
 107. The biosensor device of claim 71, wherein: the measurement circuit applies a voltage to the working electrode terminal and a voltage to the counter electrode terminal during the assay; and a current flowing through the first line is substantially equal to a current flowing through the third line. 